Thing is that in that case he didn't try to absorb but to free them. If he had done the former, he wouldn't succeed in his aim. Plus, we can't even know if back then he could handle that much of chakra in the first place. However, in our current situation, the Bijuu don't need to take a form in the "real" world because they already have one. The thing is that they're simply trapped. For that reason, if he absorb their chakra, he won't hurt then and at the same time he will be able to free them. I don't say absorb them all, as entities, bu rather what Sasuke already stole.
That's a nice contradictory sentence XD
Well, for one we don't even know if they're about to collide, he may simply redirect one of them to break Chibaku tensei.