I've used the Uni-syn device to do a dual carb tune & I found having 2 helped, and it also helped to have an assistant, (& a pair of short "stacks" to raise them, if your air horn has a bolt in the middle.) To do my VW with twin Solex carbs I used to use tomato sauce cans with the ends removed & some gasket foam on the ends. Tiny air leaks will goof the readings, so however you fit it up, get it to seal. With a dual Weber (de Brazil') carb set I didn't need the cans, as they have a horn like the B&B.
It's harder to balance a multi-carb engine with these but I've used a bank of ported vacuum gages to do 2, 3, & 4-carb engines & that usually works better (especially for sidedrafts, though the Uni-syn works with a sidedraft too.)
The thing is that you want to see how the other carbs react too, when you twiddle the mix on the carb with the Uni-syn on it, (though the difference may not be so evident on log-manifold or plenum setups as a straight side-draft setup.) With only one device you can tell something but IMO you can't tell enough.
Say you have a 4-carb setup: If you see carb 1 gets a different reaction than carb 2, when you twiddle carb 3, but carb 4 gets the same reaction as 1 & 3, then you know something's up with cyl 2.
BTW, I thought those Brazilian Webers were real good carbs, and 2 of them would supply a 100 HP+ VW engine. All the jets were removable, so tuning was very flexible. Mine did not leak like the Solex carbs did.