i don't think it would cancel it out. it wont work that way. Shinra tensei PUSHES, while kamui ABSORBS .
According to me , If nagato PUSHES with some force, kamui would simply absorb that Force.
Two forces MUST BE IN OPPOSITE DIRECTION, to cancel out each other.
(in your theory, they are in same direction.)
BUT yea, nagato can still use bansho tenin (with which he attracts objects towards him) but neither nagato or madara have so much pulling power that they overcome kamui itself.
And if they do have such a powerful bansho tenin, still it would NOT CANCEL KAMUI, they would just attract kamui to itself.
(IF YOU HAVE SOME PROBLEM UNDERSTANDING, just see 2 vaccum cleaners, first set them on 'absorb'/'vaccum' mode (that would be kamui and bansho tenin) and then let one on 'push' 'mode and other remain at vaccum mode(shinra tensei and kamui) )