In our state, you might not be able to adopt both at the same time either. Be sure to check your state/agency laws. Piggy-backing on what aclee mentioned:
Adopting an 18month old can be tough. I know you have the medicals; but be sure to know indepth what the emotional aspects of this toddler are. It won't be the medical issues so much; as it will be the emotional ones. And, the emotional ones could be very trying.....
As for having both.....both of these babies will require a lot of time and patience and time and patience and---did I mention 'time' and patience?? LOL
Not saying you couldn't do it; heaven knows a lot of successful adoptions have taken place where this type of thing is the case. But, make sure you have a lot of support; make sure you're up for the challenge and you'll probably be fine. No, I know you'll be fine if this is what's supposed to happen, and you and your dh feel you can do this.
Of course, be sure to alert the possible emom of having the 18month old. (This might be something that would appeal to her anyway....having a ready-made sib, KWIM?)
And, best of luck. I know you'll keep us updated...
Sincerely,
Linny