I dont think its possible to get both Hannah and Penelope in the dressing room, I followed the WT (not been updated but still) and didn't get Penelope to do anything. I think parts of Hannah's route blocks parts of Pen's route as I never got certain scenes on her route while doing Hannah's route.
That
may be true, but idk.
Nobunaga oda had said "or" in their post, so I assumed
neither option was working for them. I have saves for playthroughs using various routes. When I have the option for walking in on either of them
and I've been nice to both, Penelope basically says "not here, not while Hannah's next door": she doesn't kiss the MC, but she also doesn't kick him out. Hannah will kiss him and say, "I like you too, but not here, not while Penelope's around". So I do think they at least partially block each other, but you should still be able to have the MC walk in on either in the same playthrough w/o their being pissed at him.
When I had the MC be an asshole to Hannah (forced her to dive in the pool, etc.) and Penelope (told her to take the bus, etc.), later at the changing rooms both were irate and kicked the MC out immediately after cussing him out.