Here are a couple of articles you might find useful:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

http://www.thecarburetorshop.com/Triple1barrels.htm

Do it right and you will like the results. Using 2 (or 3) carbs you just happen to have might NOT give you the desired results.

A three-carb manifold with two dummies will not perform as well as a single carb set-up.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop