It's a bit of a personal thing really - what one person likes the next might loathe. For me I would take my 'normal' dose of both but it's not like I'm unfamiliar with either. The main thing I notice is that 2C-B can add quite a bit to feeling kinda hot so have cooling things and fluids on hand. For a first time - and with no prior 2C-B experience - I'd recommend not combining them at all frankly. If you really want to no more than the one 2C-B tab and perhaps half your normal MDMA dose? It really is hard to suggest anything other than trying 2C-B on its own first in a situation like that to be honest though. 2C-B is euphoric as hell so doesn't need MDMA to keep it on the perky and cheerful side.
EDIT: Good point, Joe. There is a particular synergy between the end of an MDMA session and 2C-B to pick it up again. That would be a better option than a straight-up double drop type combo if you really must combine them first time. As he says, wait until the MDMA is tailing off and drop a 2C-B tab and wait for the pretty things.