I do completely see that MI, we have a similar situation with Youth Theatre, which dd2 is due to start next year in Y6. However, ds is thinking about doing Drama GCSE and as he doesn't have the confidence to do the School Plays, he would feel more comfortable at Youth Theatre. The school has auditions on Sunday, but apparently the whole play has to be done in a Welsh accent, so he has decided not to go for it! The Youth Theatre has always been the 'thing' dd2 is going to do.
So the thing you have in your favour is that dd1 doesn't mind, which is great. Would they be in the same session (ours seem to be done by year group)? Would dd2 be better at it than dd1? THIS IS A BIG ONE 
I would be tempted to let dd1 have another term or so on her own there and then introduce it to dd2.....doesn't solve it but gives everyone a bit more time to get used to the idea.
Rose - love that range - I've often lusted after Alice Temperley, but always assumed it was out of my range. Those tops are great....
Glad you are hunkering down, Crem. I can't tell you how much I'm looking forward to my two non-working, non-children days tomorrow and Friday. I shall skip through my jobs in whatever order I choose.....