You don't need two separate double breakers. Just use a single double
breaker for both runs. The blacks on both runs go to one phase, the
reds go to the other phase.

I think you can get rid of some breakers if you like by combining two
runs into a single circuit (as long as you are careful about hidden
split circuits (where they didn't use a double breaker as they should
have). Combining two 15A circuits from one of the phases into a single
15A circuit on the same phase will in most cases be fine. Most circuits
in your house will have less than a few amps at all times (unless you
have a penchant for plugging in and turning on ALL your appliances and
lights at the same time -- and even so... last time I tripped a breaker
due to simply too much draw was at least a decade ago).

Check with some others first, and think about the circuits a little
before doing anything. Sometimes there are reasons circuits are split:
maybe they really do need that much current (space heaters, AC, larger
loads, etc.), or are required to have their own circuit for code
reasons, etc.

-Kevin