Splicing together two films starring titular protagonists to make a cohesive third film between them has been a big fad for the past year. One of the best editors out there is AMDS, who can make you believe you are watching a Hollywood production that actually never existed.
Consider the case of Arnold Schwarzenegger Vs. Sylvester Stallone, which gets pretty hilarious around 5m30s.
He even has sequels to his wildly popular mashups, such as Terminator vs. Robocop, 1 & 2:
Check out his other stuff on Youtube.