They are both /k/ ooooldfags, Flandre and Mugi (biological woman who had Flandre's kids) met in Nuggetfest well over a decade ago and hit it off, Flandre did however sadly formally troon out after years of teehee "totally ironic" crossdressing.
There also was a very old semi nsfw video going around of Flandre, Wirty (another North East turbo faggot) and a couple other fags dressed in toohoo cosplay having pretend sex, rumor is they all sucked eachother off during one Nuggetfest years ago, NF would be so great if only they banned faggots.