Board Thread:Theories and Speculation/@comment-26388300-20150612203442/@comment-734582-20150614130502

The important part is that the jutsu needs to be undone. before the summoned is fully returned to the pure world, they can attempt to Edo Tensei themselves. Notice that with Orochimaru, the jutsu was never undone. Hence, they never had a chance to Tensei themselves.

Secondly, Madara possessed the Rinnegan when under Tensei, which gives the user great insight into any technique like a sharingan.