I do normally do both sides or at least two armies that could fight each other. Recently we’ve been working with rules that cover a wider range so my main opponent can do the same – but different.
For WW1 I have a large German 1914 army plus British & Belgians. He has a smaller German army plus a large French 1914 army. He then did Russians to face his Germans but, as I didn’t fancy Austrians I moved on to RCW (where his Russians work too).
For RCW I did the core red & white forces (with other bits still to complete) & he did Poles
Now we’ve got into Irregular Wars (our own modified version for later dates) and we have bits of all sorts between us from late 17C to FIW.
All those are 10mm so, if I want an opposing force, I had to do both sides as the local group I game with only do WW2 in 10mm (as well as almost every other scale you can think of) – at least until Jeff got the 10mm bug.
Smaller than that and everything I have is both sides.
Tony of TTT