I looked it up in the manual, and it turns out there's an
advance-consumption-per-rp table which (assuming it's implemented) does
more or less what I thought would be necessary. To force the side to
choose ground or space battles, you have an advance for each which
requires a point of "research commitment", and you only get one point of
that, so you have to choose one or the other. To force choosing one or
the other first, but eventually allow them both, make it require 100
points of research commitment, give the side 100 points, but then make it
hard to produce additional points... so they can eventually make another
100 and take the other path, but it'll take a while.