It's not a requirement, but they are both the same age and the other side will be needing replacement soon. Do them together and one alignment covers them both. Do both, and you will need two alignments. In addition, this is one day in the shop vs two.
OP, If you had two front tires that were both close to the end of their life and one of them experienced some sort of failure, would you buy one new tire only or would you buy two new tires ? Rob43