I just did this combo last weekend, it was fucking magical
Did 125mg MDMA, then 2-3 hours later 25mg 2C-B. (both oral) If you time it so that the 2C-B kicks in just as you're starting to come down off the MDMA, it works really well in getting you high again at just the right time. For me 2C-B takes an hour to kick in though, so keep that in mind.
If it's your first time with 2C-B then 25mg might be a bit too much. Personally I wouldn't go any lower than 15mg though. You want to get some nice visuals and effects, and I find with lower than 15mg you don't get that and just you sit there waiting for "it" to happen. I'd recommend 15-20mg if you have previous experience with other psychedelics.
It will be lovely at a festival. Just make sure you're surrounded by nice people. Maybe find a quiet place where you can hide for a bit if you just want to enjoy and take it all in. I find that when the 2C-B is peaking I don't want to dance as much but just stand there amazed at the sounds and visuals
Comedown is no worse than usual, perhaps sleeping is a bit more difficult than after just MDMA. I always have loads of CEVs when I try to get to sleep hours later, that can be a bit distracting, but it's actually quite fun.
Enjoy
