Mixing two beers - different gravities

I have a bit of a conundrum. I currently have 12 gallons of Czech pilsner. They are divided into 6 gallon batches. After 4 weeks at 52 F, one has a gravity of 1.029 the other has a gravity of 1.015. The lower gravity batch is crystal clear, and probably ready to lager.

I don't know why they differ. Perhaps I oxygenated one better than the other, or topped up with more water in one than the other, I don't know...

Should I purchase some more yeast, build it up, decant, and pitch into the one that is stuck? Should I rack the stuck one onto the other cake? What will I get if I mix them?

Maybe the one was slightly underpitched and got too cold, too quickly and the yeast went dormant? Maybe warm it up a little and see what happens... you could always pitch a dry lager yeast to try and finish it up.

The unfinished one is very estery. I think? It tastes a bit like the Belgians I make, there is a definite spice note to the stuck beer. I think the yeast may have just became very angry, and died. I should point out that both were underpitched (2 vials built up to 2 liters). There was no way I was going to make 3.23 gallons of starter!

Don't make your yeast angry. You won't like them when they're angry.

I looked up the definition of SG (its basically just density with a small correction), and the solution to this problem is pretty simple. If I mix equal quantities, I can just average the readings, assuming no further attenuation. This would be 1.022, a little high.