I've recently attempted to use two monitors with my laptop's single DisplayPort. After enabling two Ubuntu 14.10 ppa repos (specifics below), I managed to get them recognized by xrandr and unity-control-center, but when I enable them I get what looks to me like a crash in the Intel driver:

==> Xorg.0.log <==
[   103.213] (EE) intel(0): failed to set mode: Invalid argument [22]

==> syslog <==
Nov 19 19:20:56 dib kernel: [  103.208129] ------------[ cut here ]------------
Nov 19 19:20:56 dib kernel: [  103.208183] WARNING: CPU: 2 PID: 1758 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_ddi.c:850 intel_ddi_pll_select+0x7f/0x3c0 [i915]()
Nov 19 19:20:56 dib kernel: [  103.208186] Invalid DDI encoder type 11
Nov 19 19:20:56 dib kernel: [  103.208189] Modules linked in: ctr ccm arc4 uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core v4l2_common videodev media iwlmvm btusb intel_rapl mac80211 x86_pkg_temp_thermal intel_powerclamp coretemp snd_hda_codec_hdmi kvm_intel snd_hda_codec_realtek snd_hda_codec_generic iwlwifi kvm snd_hda_intel snd_hda_controller snd_hda_codec snd_hwdep snd_pcm rfcomm rtsx_pci_ms bnep cfg80211 joydev bluetooth serio_raw memstick 6lowpan_iphc thinkpad_acpi lpc_ich nvram shpchp snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer snd parport_pc mei_me soundcore intel_smartconnect ppdev mei mac_hid lp parport binfmt_misc dm_crypt rtsx_pci_sdmmc crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd psmouse i915 ahci libahci e1000e rtsx_pci i2c_algo_bit drm_kms_helper ptp pps_core drm wmi video
Nov 19 19:20:56 dib kernel: [  103.208286] CPU: 2 PID: 1758 Comm: Xorg Tainted: G        W     3.16.0-25-generic #33hf73386v20141106b1-Ubuntu
Nov 19 19:20:56 dib kernel: [  103.208289] Hardware name: LENOVO 20AQ005YUS/20AQ005YUS, BIOS GJET75WW (2.25 ) 03/28/2014
Nov 19 19:20:56 dib kernel: [  103.208292]  0000000000000009 ffff880210b8faf0 ffffffff8178015c ffff880210b8fb38
Nov 19 19:20:56 dib kernel: [  103.208298]  ffff880210b8fb28 ffffffff8106fd7d ffff880212948000 000000000000000b
Nov 19 19:20:56 dib kernel: [  103.208303]  ffff88020e890000 0000000000083d60 ffff880035dd8e00 ffff880210b8fb88
Nov 19 19:20:56 dib kernel: [  103.208309] Call Trace:
Nov 19 19:20:56 dib kernel: [  103.208320]  [<ffffffff8178015c>] dump_stack+0x45/0x56
Nov 19 19:20:56 dib kernel: [  103.208328]  [<ffffffff8106fd7d>] warn_slowpath_common+0x7d/0xa0
Nov 19 19:20:56 dib kernel: [  103.208334]  [<ffffffff8106fdec>] warn_slowpath_fmt+0x4c/0x50
Nov 19 19:20:56 dib kernel: [  103.208370]  [<ffffffffc050cc4f>] intel_ddi_pll_select+0x7f/0x3c0 [i915]
Nov 19 19:20:56 dib kernel: [  103.208401]  [<ffffffffc04e8d62>] haswell_crtc_mode_set+0x12/0x40 [i915]
Nov 19 19:20:56 dib kernel: [  103.208433]  [<ffffffffc04f8176>] __intel_set_mode+0x6d6/0xab0 [i915]
Nov 19 19:20:56 dib kernel: [  103.208465]  [<ffffffffc04fb456>] intel_set_mode+0x16/0x30 [i915]
Nov 19 19:20:56 dib kernel: [  103.208495]  [<ffffffffc04fc56a>] intel_crtc_set_config+0xa9a/0xe30 [i915]
Nov 19 19:20:56 dib kernel: [  103.208521]  [<ffffffffc03d0571>] drm_mode_set_config_internal+0x61/0xe0 [drm]
Nov 19 19:20:56 dib kernel: [  103.208544]  [<ffffffffc03d4503>] drm_mode_setcrtc+0x283/0x580 [drm]
Nov 19 19:20:56 dib kernel: [  103.208562]  [<ffffffffc03c4a4f>] drm_ioctl+0x1df/0x680 [drm]
Nov 19 19:20:56 dib kernel: [  103.208575]  [<ffffffffc06864a2>] ? snd_seq_client_enqueue_event+0x52/0x140 [snd_seq]
Nov 19 19:20:56 dib kernel: [  103.208581]  [<ffffffff811f4c28>] do_vfs_ioctl+0x2c8/0x4a0
Nov 19 19:20:56 dib kernel: [  103.208587]  [<ffffffff81084a69>] ? restore_altstack+0x19/0x30
Nov 19 19:20:56 dib kernel: [  103.208592]  [<ffffffff811f4e81>] SyS_ioctl+0x81/0xa0
Nov 19 19:20:56 dib kernel: [  103.208598]  [<ffffffff817881ad>] system_call_fastpath+0x1a/0x1f
Nov 19 19:20:56 dib kernel: [  103.208600] ---[ end trace c91658f1a3f643b4 ]---
Nov 19 19:20:56 dib kernel: [  103.208604] [drm:intel_crtc_set_config] failed to set mode on [CRTC:9], err = -22
Nov 19 19:20:56 dib kernel: [  103.208607] [drm:intel_modeset_affected_pipes] set mode pipe masks: modeset: 0, prepare: 0, disable: 2
Nov 19 19:20:56 dib kernel: [  103.208620] [drm:intel_connector_check_state] [CONNECTOR:15:eDP-1]
Nov 19 19:20:56 dib kernel: [  103.208624] [drm:check_encoder_state] [ENCODER:14:TMDS-14]
Nov 19 19:20:56 dib kernel: [  103.208627] [drm:check_encoder_state] [ENCODER:22:TMDS-22]
Nov 19 19:20:56 dib kernel: [  103.208629] [drm:check_encoder_state] [ENCODER:24:DP MST-24]
Nov 19 19:20:56 dib kernel: [  103.208631] [drm:check_encoder_state] [ENCODER:25:DP MST-25]
Nov 19 19:20:56 dib kernel: [  103.208633] [drm:check_encoder_state] [ENCODER:26:DP MST-26]
Nov 19 19:20:56 dib kernel: [  103.208634] [drm:check_encoder_state] [ENCODER:28:TMDS-28]
Nov 19 19:20:56 dib kernel: [  103.208636] [drm:check_encoder_state] [ENCODER:30:DP MST-30]
Nov 19 19:20:56 dib kernel: [  103.208638] [drm:check_encoder_state] [ENCODER:31:DP MST-31]
Nov 19 19:20:56 dib kernel: [  103.208639] [drm:check_encoder_state] [ENCODER:32:DP MST-32]
Nov 19 19:20:56 dib kernel: [  103.208641] [drm:check_crtc_state] [CRTC:6]
Nov 19 19:20:56 dib kernel: [  103.208650] [drm:check_crtc_state] [CRTC:9]
Nov 19 19:20:56 dib kernel: [  103.208652] [drm:check_crtc_state] [CRTC:12]
Nov 19 19:20:56 dib kernel: [  103.208748] [drm:drm_mode_getresources] CRTC[3] CONNECTORS[7] ENCODERS[9]


System: Lenovo T440s
Video adapter:
00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 0b) (prog-if 00 [VGA controller])
	Subsystem: Lenovo Device 220c
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 62
	Region 0: Memory at f0000000 (64-bit, non-prefetchable) [size=4M]
	Region 2: Memory at e0000000 (64-bit, prefetchable) [size=256M]
	Region 4: I/O ports at 3000 [size=64]
	Expansion ROM at <unassigned> [disabled]
	Capabilities: <access denied>
	Kernel driver in use: i915
Monitors: 2xDell U2413

Kernel: 3.16.0-25-generic #33hf73386v20141106b1-Ubuntu SMP (from http://ppa.launchpad.net/dgadomski/linux-mst/ubuntu/)
Driver: 2:2.99.916+git20141112.f9f85b88-0ubuntu0sarvatt (from https://launchpad.net/~xorg-edgers/+archive/ubuntu/ppa)