Does anyone step in to fill the holes in your roster?
Solution 1:
There is no permanent replacement for either. If they die, they die.
There are a few missions where a character who is key to the narrative is replaced, not as a squad member, but in terms of cutscenes and dialogs by another, related individual who you met in Mass Effect 2.
That said, even if both are dead, you're overlooking a squad member. You still have more options than the Survivor, Liara and James.