how much head flow to justify dually carbs?

Working with multiple carbs is an art.
I'm 52yo and been handy with it my whole life.
If you or any of your mates are inexperienced with multiple carbs then be prepared for a lot of pain before you see any gain over a single (easy to work with) 4bbl.
When you go multiple you'll be running more cfm than what sounds right for a single 4bbl.

Example : my 440 barracuda has 400hp and had dual 500 edelbrocks on it originally.
A single 850dp would have been more than adequate.
I then swapped out to dual 600vac holleys.
Since then I've built a home made mechanical sixpack with dual 350 2bbls as the primaries and a single 650 2bbl as the secondary(thirdary?)which totals 1350cfm but comparing the 2bbls to the 4bbl vacuum draw they come in around 950cfm total.

Tuning is the biggest issue.
Make sure you've got someone who knows what they're doing.

And just out of interest : many years ago I set up 6x 1 1/4" SU's on a six cylinder inline Ford engine for a mate.
Along with many triple weber,triple SU and twin downdraught carbs on slants and aussie hemi sixes.
Even ran alcohol through webers.