If Russia is strong and the ERE is weak, the latter is definitely getting subsumed within Russia, especially if the latter owns Constantinople/the Dardanelles.
If Russia is strong and the ERE is strong, they are not going to unite and will likely go to war with each other. It's not like the Rus' have never attacked Byzantium before, and Russia's quest for warm water ports will almost-inevitably bring it into conflict with nations to its south.
If the ERE is strong and Russia is weak, nothing much happens. I can't see Byzantium wanting a vast, unruly 'wasteland' full of barbarians in their Empire, especially as it means bringing more Russian noble families (and thus contenders for the throne) into Constantinople.