The following XML sends a Group command sequence for the specified Group and Command. I tested this through the SHN Utility Control tab and confirmed that my EZSERVE as Controller Unit 20 Scene controlled both Responders with the single XML command. The expected Group Broadcast message followed by a Group Cleanup Direct message for each responder was issued. Two responders are in this particular Scene, an ICON inwall Switch and an EZIO2X4 Output Relay. Of course Insteon supports more responders in the same scene. This test was run on an EZSrve running image 1.60. Group commands are how native Insteon devices control multiple responders, provides near simultaneous On/Off operation with the ability to have each responder react with its own Bright level and Ramp rate.
I have no definitive information on XML command spacing.