unlikely, very very unlikely, for one thing they're both major powers nether would dig being the junior partner in a crown union (having the King living in the other nation) they're far apart making it hard for the King to do his duty in both nations, religion is maybe the biggest issue, the Spanish Monarchs are the "Most Catholic Monarchs" Russia has even more of an issue because the Tsar had a major role in the Russian Orthodox Church.
the only way I see it happen is a Napoleonic thing, Napoleon makes Joseph Tsar in 1812 as well King of Spain.