Most people interested in the history of Eastern Europe probably know (or at least they should) that such a union has been considered several times in actual history. Several attempts/suggestions were made, including Ivan the Terrible being pretty close to being elected the monarch of the PLC, Polish prince Vladislav actually having been elected the tsar of Russia (later expelled though), and several others.
This should obviously be extremely difficult to achieve, due to lots of underlying contradictions in religion, political systems et cetera. All these factors actually prevented it in the real history.
Can see several ways to this:
1)PLC conquers Muscowy or vice versa, and after a very long period of occupation (like 50-100 years or more), constantly quelling rebellions, enacting lots of appeasing decisions having a chance to get access to a decision to form a proper union.
2)A veeery religiously free-minded Muscowy and PLC, having similar political sliders, being in a personal union, instead of having a chance to inherit, have a chance to form PLMC/PLRC.
3)PLC being able to form PLRC after having assimilated a number of Russian principalities (not necessarily Muscowy) OR Lithuania forming Lithuanian-Russian commonwealth alternatively to PLC (possibly with an option to later have a union with Poland also). Based on real history, prior to Muscowy more or less unifying Russia through conquest and politics (and a good deal of time after that), Lithuanian Grand Duchy (being mostly Slavic despite the name) has been considered an alternative center of power in the ex-Kievan Rus'. Western Russian principalities had historically as much chance to be unified by GDL as by MDL. Again, this would require lots of religious/political reforms, generaly huge losses of stability and control, appeasing of the population and nobility et cetera.
All of this probably should only be possible in the 1st half of the game (prior to about 1650). Later it just seems too unrealistic if we stick to the idea of "different but similar" history (as it is in the EU series mostly) as opposed to free-for-all alternative history.
As a PC seems to be unlikely to achieve it on its own, to give us some chance to see it other than playing it out ourselves, PCs should have a chance (like about 10%) to acquire a series of missions leading to one of these outcomes.