I am so sorry to hear about your troubles. I guess you just have to ask yourself how badly you want another child? If you really want another one and think you will always regret not trying, then I say go for it. As PP said, maybe you could check into adoption?
I have 2 kids, a boy and a girl.
I like having just two because you can handle them on your own. You aren't too outnumbered if you take them somewhere, kwim?
The financial obligation of a third, WOW! Three college tuitions, eek!
I have a boy and a girl, so I don't have to worry about trying to get that girl or boy,. (not sure if this applies to the 2 you have or not)
They will always have that special bond with each other without having a favorite brother of sister and leaving one sibling out of things.
I cannot imagine imagine juggling more stuff--tee ball, soccer, gymastics, etc.
I feel like I get to spend quality time with both of them without having to divide my attention any further.
I am TOO OLD and tired to have any more kids. lol
Whatever, your decison I wish you luck and peace with it.