Is it possible to unify two proposals ?
As a consequence, the new unified proposal would be proposed by two people (instead of one), and all the statistics (followers, example questions, views...) should be added.
See here for a concrete example.
According to the link from JohnB there is some technical mechanism for it, so the literal answer to your question would be "yes".
But what would be the use of such merge? Anyone is free to join another proposal, so it's better to close one of them as duplicate. Another problem is when some user have asked 5 questions on both proposals. 5 of them need to be deleted, but which of them?