We too were faced with this decision, back when we were matched with Emma's bparents. We had just matched with them "officially" and a second expectant mother who had first contacted us shortly after (within a week) we'd had the first contact from Emma's bmom then said she also wanted to place her baby with us. Their duedates were a month apart, though Em wound up being born early which would've made her a week older. We were up front with both women that there was another possibility and asked BOTH how they felt about the "aritificial twins" idea. Neither of them had a problem with it at all, as it turned out. Then we did some real soul searching and thinking and reading and research, and it was agonizing but we ultimately decided it wasn't right for us at that time, so we turned down the second match. Believe me, I know the what ifs that run through your mind! It was soooo tempting, especially when we were ultimately hoping to have two children someday, but in the long run we were glad we didn't do it, especially with Em being in the NICU when she was born. We kept in touch with the second woman, and she did find another afamily for her daughter, and even shared pictures of her bdaughter that first Christmas after the girls were born. We were glad that she found another family and had a good relationship with them.
We were later contacted by another expectant mother who had seen our profile online (they left it up as "hey look these folks matched" sort of advertising for a bit after Em came home) some months before, and when she contacted us, she said she knew we had already adopted, but she really felt that we were the right family for her baby and would we consider adopting her baby, who was a boy due when Emma would be 10 months old. Not quite "artificial twins" there, but still close in age. Again, we agonized about the decision but felt that for us, it wasn't the right time, and we ultimately decided to say no to this baby too.
It is sooooo hard when faced with having to make this choice. I know for us, one kiddo at a time was the best thing, but I also know other people (like our neighbors across the street) who have done quite well with two children so close together in age. There is a lot of information on the subject if you do a Google search under "artificial twinning" if you are interested in studies and whatnot.
I wish you the best of luck in whatever you decide, and I wish you peace in your decision.
Hang in there!
Heather