This is a patch I made to add options to control mergedfb behaviour, using
options similar to those present in radeon driver.
MergedXineramaSecondIsScreen0 swaps the logical Xinerama screen in mergedfb mode.
MergedNonRectangular allows to have different resolutions.
MergedMouseRestriction restrict mouse movement to display size when using non
rectangular framebuffer.

I tried to use naming conventions similar to other options. Patch is against
1.7.2 version, but it's suitable also for 1.7.3 and 1.7.4 versions..

Bye

--- a/src/i830_driver.c
+++ b/src/i830_driver.c
@@ -268,6 +268,9 @@
    OPTION_SECONDHSYNC,
    OPTION_SECONDVREFRESH,
    OPTION_SECONDPOSITION,
+   OPTION_SECONDISSCRN0,
+   OPTION_MERGEDFBNONRECT,
+   OPTION_MERGEDFBMOUSER,
    OPTION_INTELXINERAMA
 } I830Opts;

@@ -292,6 +295,9 @@
    {OPTION_LINEARALLOC, "LinearAlloc",  OPTV_INTEGER,   {0},    FALSE},
    {OPTION_MERGEDFB,   "MergedFB",     OPTV_BOOLEAN,   {0},    FALSE},
    {OPTION_METAMODES,  "MetaModes",    OPTV_STRING,    {0},    FALSE},
+   {OPTION_SECONDISSCRN0,"MergedXineramaSecondIsScreen0", OPTV_BOOLEAN, {0},
FALSE },
+   {OPTION_MERGEDFBNONRECT,"MergedNonRectangular",OPTV_BOOLEAN,        {0},  
FALSE},
+   {OPTION_MERGEDFBMOUSER,"MergedMouseRestriction",OPTV_BOOLEAN, {0},    FALSE},
    {OPTION_SECONDHSYNC,        "SecondMonitorHorizSync",OPTV_STRING,   {0},
FALSE },
    {OPTION_SECONDVREFRESH,"SecondMonitorVertRefresh",OPTV_STRING,{0}, FALSE },
    {OPTION_SECONDPOSITION,"SecondPosition",OPTV_STRING,        {0},    FALSE },
@@ -4685,6 +4691,25 @@
          }
          xfree(tempstr);
       }
+
+      /* If OPTION_SECONDISSCRN0 is true then swap screens */
+      if(xf86GetOptValBool(pI830->Options, OPTION_SECONDISSCRN0, TRUE)) {
+        if (pI830->SecondIsScrn0) pI830->SecondIsScrn0= FALSE;
+        else pI830->SecondIsScrn0= TRUE;
+      }
+
+      /* Set pI830->NonRect according to OPTION_MERGEDFBNONRECT */
+      if(xf86GetOptValBool(pI830->Options, OPTION_MERGEDFBNONRECT, TRUE))
+             pI830->NonRect = TRUE;
+      else
+             pI830->NonRect = FALSE;
+
+      /* Set pI830->MouseRestrictions according to OPTION_MERGEDFBMOUSER */
+      if(xf86GetOptValBool(pI830->Options, OPTION_MERGEDFBMOUSER, TRUE))
+              pI830->MouseRestrictions = TRUE;
+      else
+              pI830->MouseRestrictions = FALSE;
+
       if((s = (char *)xf86GetOptValString(pI830->Options, OPTION_METAMODES))) {
          pI830->MetaModes = xalloc(strlen(s) + 1);
         if(pI830->MetaModes)