Is there a formal phrase for saying that the location of an event changes between two places?

Solution 1:

I'd say it has "alternating locations of X and Y" or that it is "alternately held in X and Y".

Solution 2:

The two (feasibly more) locations are alternating or rotating venues.

You might prefer the second version if there are more than two, particularly if they host the event in a regular cycle. And if there's no pattern to the hosting sequence, you might prefer alternative.

Solution 3:

Alternating between venues yearly.

Solution 4:

The conference is held in X and Y in alternate years.

You could also mention that it's in X in even years and Y in odd years, or leave the reader to infer that from the fact that the 2014 conference is in X.