A source close to Kasich reported that the Ohio governor tried to broach the subject with Rubio as well, and the campaigns discussed it as well, before and after the March 8 Michigan primary. Kasich's team did not conduct any polling but they are also convinced if the two men had teamed up, "we would have swept the rest of the primaries."
Heh it's pretty funny - looking from Europe it looked rather obvious Trump will take it all unless other candidates drop fast and unite into one camp. He was winning because rest of votes was split among so many other candidates.
Trump would have destroyed VP Rubio even harder than he did the first time. I don't think that making your ticket more-establishment would have done much good against Trump anyway.