1. You could use OpenBRF and merge the Faction Reskin you want to basically a bigger Faction reskin containing 2 or more factions. -> thats the method mclovin wrote about
2. It might be possible to install all 4 of mine if you always use only the brf's of the Faction got replaced in the reskin
(Basically mmmaterials+mmtextures+mmaustrianuniforms+mmaustrianhats from Baden, mmfrenchuniforms+mmfrenchhats from Bayern, mmrussianuniforms+mmrussianhats from Württemberg, mmprussianuniforms+mmprussianhats from Westfalen + one of the britishuniforms brf in those reskins to fix the british officers and their pants.)
I haven't tried it but technically It could work since only my Reskin for Baden uses the mmmaterial and mmtextures.brf so that wouldn't intefere with the rest of the 3.
;well the downside would be you still can only have one faction to actually be named like in the faction reskin.
Another one (or the same) can have the Flags used in the UI. And again another faction (or the same again) will have their actual flags for the units ingame.)
3. You just don't do any of this and start to be ok with just having 1 Faction. -> worst option I guess