@Rex Coil 7
I don't know how much longer the 1260 swingarm is over the 1200 either but I do know that the stock 1260 15/40 chain is 114 link whereas the 1200 15/40 is 108 links.
So that suggests roughly 3 links longer (6 links halved). Just estimated, that is ~about~ 2 inches .... roughly so.
Is the specified wheelbase of the two bikes 2 inches or so in difference?
The point there is that I'd guess that Ducati didn't just stick two inches on the rear of the bike, I'd guess that the frames, engine placement, swingarm pivot location, and headstock placement have all been moved around to create a specific weight bias on the longer bike. That said, sticking the longer swingarm on the shorter bike will not replicate the longer motorcycle ... it will create an entirely different machine altogether. As I previously pointed out, a longer swingarm will improve "suspension compliance" which will serve to keep the rear tire attached to the road surface ... which we all know means better traction and handling in all situations (accelerating, braking, cornering). Compliance may have been at least part of Ducati's goals (rather than just sticking 2 inches on the ass end of the longer bike) ... and that can be determined by looking at how Ducati distributed the longer swingarm's increase in length.
That said ....
... I know the manuals for the Supersports, Monsters, and Superbikes all have fully dimentioned frame diagrams near the back of the manuals. Perhaps these Multistrada manuals also contain such data. If so, it would be pretty simple to check engine placement, headstock location, swingarm pivot location, and so on in comparison with each of the two Multistrada models (the one with the shorter swingarm compared to the one with the longer swingarm).
That would be deeply valuable information to study before attempting to attach a longer swingarm to the shorter bike.
Some of that extended swingarm length may have also been applied towards moving the swingarm pivot closer to the countershaft sprocket's center without moving the weight bias too far forward. The closer the swingarm pivot is to the countershaft center the less the bike is upset during acceleration and engine braking, making for a more stable motorcycle when throttling up and throttling back. The closer the swingarm pivot is to the countershaft sprocket center also means that the chain's length will vary less throughout the entire rear wheel travel, meaning a bit more chain tension may be used without worry of binding the rear suspension since the chain's tension will vary less throughout the rear suspension's travel. A bit less chain slop provides smoother throttling up and down without ~jerking~ the bike and upsetting the bike's attitude and destabilizing it, which is especially advantageous while cornering.