When upgrading to two carbs, airflow and manifold vacuum for each carb drops.
Not exactly in half but low enough to give a rich condition for each carb at all times.
Some try to compensate with smaller jets, weaker accelerator springs, etc.
For a 2x1 setup under 3500rpm, I believe the 235 and 261 runs best with 216 carbs.
I run 2 Carter W-1s on my 235 and really like them.
I went through several Rochester Bs of various sizes - the only one that didn't leak was NOS.
Carters are better designed, leaner, cleaner and better responding.
Chevy in the 40s used Carter carbs, but switched to Rochesters in the 50s.
Carter continued making upgrade carbs for most engines on the market, well into the 60s, maybe longer.
Stromberg carbs are another great choice, but pricier than Carters.
Always use a 'Uni-Syn' carb synchronizer tool, when going with multiple carbs - amazing what difference it makes.