A chain is only as strong as it’s weakest link. The entire GI system will be probed for weakspots. If the sphictor to sphinctor seal is stronger than say, one participant’s colon, even with equal pressure, the weakest spot will perforate and burst. The gas of both participants will then flood the available cavity and expand until either equalization of the outward gas and inward retailing wall of fat and skin amd whatnot, or pressures will rise until another breach is found.

While quite correct, this is of course all theoretical. There are many details that would need accounting for. I have a research proposal I’m happy to share if anyone has any funding.