One thing that helped me is to send one sim home. There should be option to on the select sim in lower left. Once they go home, there should now be an option there to "switch control to" them, which should bring you to them at the home lot. Then switch control back to sim that is still in Batuu. Once there go to that same button again and choose to have the sim that is at home to "join vacation". And voila!.. there they are.. both on vacation in Batuu! I don't know how this works with more than 2 sims on the vacation. Probably have to send all home except one. But maybe another option is to just send one to start and then have all you want to attend "join vacation".
Another problem that happens is if at any time you go to "plan Batuu outfit" in the dwelling, when you come out, usually both sims are grayed out. To fix this, have another sim stay home so that whenever a gray-out happens, you can switch control to them and then right back to one of your sims in Batuu and the gray-outs should be gone. Or you could probably do the sending sim home, switch, switch, join thing again.
Well this is just completely nuts that this is how to get the game to work correctly, but that's what it is.