I say, only you can answer that question, not anyone here. You know what your body can take and handle, and we're not the ones that would have to take of of the baby. I can easily say "sure, go ahead and have it", but in the end, if you can't handle, you're stuck with the problem of carrying around two babies, while I'm only dealing with one. But I say, discuss it with your husband, and your doctor. And on my advice note, I would say wait, you're first one is still a baby, let him remain the baby for a while, because I've seen it first hand, as soon as the other baby comes in the picture, the first one will "turn back into a baby" and want to be babied and held, and you said yourself, you can't carry two babies and once.
Just go with your gut instict, I'm sure you'll pick the right one.
Good luck to you!
T.