Two options (short of just being upfront about it, which more women are open to than we think):
-One, straight up, guaranteed ONS. Like on a holiday / trip out of town with your friends. No communication before or after. Job done.
-Have a very casual side dish, again who lives out of town, or at very least, doesn't know here you live and doesn't go where you and your gf go, EVER. Maybe a younger woman or free spirit type, who isn't looking for anything serious. Strap up.
For me, the social logistics for more than one at a time are too difficult to manage in the long term. And women work out what's going on in the end if there's something going on for more than a few weeks. The subtlest changes won't go unnoticed. The odd one night is the preferred option in that sense.
My last two GFs I met basically at the same time, and had short term 'exclusive' relationships with each one after the other, which sort of overlapped, twice (long and convoluted story).
However, one time early on, one of them phoned me on a Sunday morning when I was in bed with the other.... Nothing came of it, but an amateur move on my part. Phone should just be on complete silence, excommunicado at times like that. Getting rid of all lock screen noti's is also a good idea.
Anyway, there's just too much potential for hassle and drama, so I tend to stick simply to exclusive or not nowadays. And without taking the moral high ground, as the saying goes, we don't know what we've got til it's gone. Be smart and be careful.