How do you set the resolution for TGX and TGX+ framebuffers when they are used
as the second framebuffer on a system?

You have to set the resolution at boot time in the nvramrc. Here is a script 
that will set up the nvramrc so  that it initialises the resolution to 
1280x1024 @ 76Hz.

Note: This script is modified from one in Sun document No: 802-5011-10 Platform
Notes: SMCC Frame Buffers, available in the Answerbook volume 'Solaris 2.5 on 
Sun Hardware' - search for the string "TurboGXplus Frame Buffer".

      #!/bin/sh
      # Configure the entries for "cgsix@0" and "cgsix@1"
      # to refer to each of your frame buffers.
      # NOTE: 
      # The actual full device path/names should be determined using 'show-devs'
      # which will vary from hw platform to hw platform.
      # If you are NOT using the correct path to the device, this script will
      # not work.
   

      eeprom fcode-debug\?=true
      eeprom nvramrc='probe-all
      " /iommu/sbus/cgsix@0" select-dev
      r1280x1024x76
      " /iommu/sbus/cgsix@1" " set-resolution" execute-device-method drop
      device-end
      install-console
      banner
      '
      eeprom use-nvramrc\?=true

The script presumes sun4m architecture; for sun4c change /iommu/sbus to /sbus
The other information you will need to run the script is which sbus slot the 
second frame buffer card resides in - ie the one that will be used as the 
second head. In this instance, it assumes a cgsix frame buffer in sbus slot 3.

The script has the disadvantage that it makes the second TGX+ the console frame
buffer. You may not want this; for example if you have a SS20SX or Ultra Creator
with an additional TGX+, you may wish to set the resolution of the TGX+ while 
maintaining the SX or FFB as the console. 

In which case, try this script instead. It should set the resolution of the 
TGX+ to 1280x1024@76 while maintaining the default console.

      #!/bin/sh
      eeprom fcode-debug\?=true
      eeprom nvramrc='probe-all
      install-console
      banner
      : vsetup " 135000000,81128,76,32,64,288,1280,2,8,32,1024,COLOR,0OFFSET" ;
      vsetup 4
      " /sbus/cgsix@2" " override" execute-device-method drop
      '
      eeprom use-nvramrc\?=true

This is another alternate script that can be used to set up the resolution
of a second TGX+/cgsix framebuffer.

#!/bin/sh
#
# This is a script to set a TGX+ graphics card supporting
# the second monitor in a two-monitor configuration to
# a resolution of 1280x1024x67. Run the script in the
# Bourne shell and then reboot.
#
eeprom fcode-debug\?=true
eeprom nvramrc='probe-all
: vsetup1 " 117000000,71691,67,16,112,224,1280,2,8,33,1024,COLOR,0OFFSET" ;
vsetup1 4 " /sbus@1f,0/cgsix@2,0" " override" execute-device-method drop
device-end
install-console
banner
'
eeprom use-nvramrc\?=true
touch /reconfigure


Note: The number 4 in vsetup 4 refers to the sense code of the monitor. For
an incomplete list of sense codes, see below.
Again, please make sure that /sbus@1f,0/cgsix@2,0 is the correct full
pathname of the second framebuffer that comes up in show-devs or probe-all
from the <ok> prompt.

Here is the appropriate voodoo for several supported resolutions. Note also that
this will only work for the TGX or TGX+. 

      1024 x  768 at 60 Hz:
      	64125000,48286,60,16,128,160,1024,2,6,29,768,COLOR
      	
      1024 x  768 at 70 Hz:
      	74250000,56593,70,16,136,136,1024,2,6,32,768,COLOR
      	
      1024 x  768 at 77 Hz:
      	84375000,62040,77,32,128,176,1024,2,4,31,768,COLOR
      	
      1152 x  900 at 66 Hz:
      	94500000,61845,66,40,128,208,1152,2,4,31,900,COLOR
      	
      1152 x  900 at 76 Hz:
      	108000000,71808,76,32,128,192,1152,2,4,31,900,COLOR,0OFFSET
      	
      1280 x 1024 at 67 Hz:
      	117000000,71691,67,16,112,224,1280,2,8,33,1024,COLOR,0OFFSET
      	
      1280 x 1024 at 76 Hz:
      	135000000,81128,76,32,64,288,1280,2,8,32,1024,COLOR,0OFFSET
      	
      1600 x 1280 at 76 Hz:
      	216000000,101890,76,24,216,280,1600,2,8,50,1280,COLOR,0OFFSET

And this is what the numbers actually mean, taking 1280x1024@67 as an example: 

      117000000       Pixel frequency or dot clock    (in Hz)
      71691           Horizontal frequency            (in Hz)
      67              Vertical frequency              (in Hz)
      16              Horizontal Front Porch          (in pixels)
      112             Horizontal Sync Width           (in pixels)
      224             Horizontal Back Porch           (in pixels )
      1280            Horizontal Displayed pixels     (in pixels)
      2               Vertical Front Porch            (in lines)
      8               Vertical Sync Width             (in lines)
      33              Vertical Back Porch             (in lines)
      1024            Vertical Displayed lines        (in lines)
      COLOR           Color monitor flag
      0OFFSET         No sync pedestal flag

Monitor sense codes (incomplete list), check FE Handbook for more information:

0 is denoted by a link (a 100 ohm resistor) to pin 4 (Ground, or Sense Return).
1 is denoted by the absence of a link - the pin is unconnected.

      3 8 9 (pins)
      2 1 0 (bits)
      1 1 1	7 - No monitor connected
      1 1 0	6 - Low cost 17" 
      1 0 1	5 - Sony 16"
      1 0 0	4 - Sony GDM20 series (with built-in lead)
      0 1 1	3 - BNC Adaptor, Hitachi 19", old Sony 16"
      0 1 0	2 - 
      0 0 1	1 - 
      0 0 0	0