MarshalBraginsky
Banned
A British-Russian Personal Union. (Possibly George III and Catherine [The Act of Settlement doesn't seem to rule out an Orthodox monarch].)
Of course it's still two distinct states, but technically under the rule of one person.
How about an earlier Anglo-Russian alliance that has a PoD around the 1500s. Say Edward VI manages to reign a bit longer and Bloody Mary dies an early death. Meanwhile, Princess Elizabeth could marry Ivan in 1561 after Anastasia Romanova dies.