There are lots of examples of dual amp/cab blocks in a preset, and software could strip off the old IN/OUT blocks from one preset and tie both signal paths together at the ends.
The problem of an automated approach is that you would likely be consigning the resulting preset to have unnecessarily high CPU demands because of duplicated blocks and/or unoptimized paths, unnecessary shunts, etc. We don't have an optimizer available, so it takes a human to analyze the signal flow and make the preset work its best. And, at that point the human should just do it all.
If all you want to do is tie two presets together at the IN and OUT blocks, that's pretty easy to do by hand by exporting the blocks into the library using
Preset>Save as Blocks in Library then opening that folder and importing them individually and moving them around to reflect the old flows; Making it all work well together is what gets hard because the scenes have to be reconciled/merged/modified, and controlling it has to be organized and that's where simple software would never suffice.
Honestly, I don't think this is a practical idea and as
@mwd said it'd be a nightmare to develop and support.
