Maybe I am simply mistaken, but I thought the only time Rondo really played with Nate was during the Cleveland game, and maybe with the Nets, but I can't remember (who really wants to?). Still, I thought that was cause Nate didn't know the system enough to be the backup PG, so Doc made the 2nd unit a running unit with Rondo. Against Cleveland, that worked....for a half. Then, things started to fall apart in the 3rd quarter. MAYBE if Doc decided to play Nate as PG in the start of the 4th and gave Rondo a breather, he would've been able to finish that game stronger than he did and we would've possibly had a chance.
Anyways, come the start of March, I think Nate has clearly been running the 2nd unit as PG, while Rondo's gotten some well-needed rest. Again, like Brendan pointed out, if Rondo and Nate truly get the minutes they should, them playing together as the backcourt for a few minutes will probably be inevitable, and that's ok. Rondo, for example, has only played more than 40 minutes once in the five games (41 vs Philly) since he played those 45 minutes against Cleveland. In that same period, Nate's gotten 23, 15, 20, 13, and 14; at least 10 minutes in each game, and at least 20 in two. For what serves as games 3 to 7 for him as a Celtic, those are decent minutes to me, and assuming they increase, Rondo will get more than enough rest down the stretch and be ready for the playoffs.
Overall, I'm not worried.