I think I've figured out why the surgeon is doing both TLIF and PLIF....I still don't know why he isn't just doing a TLIF for both segments, but then it doesn't matter what I think.
The L4-L5 segment carries the majority of the spine's movement...and the TLiF puts bone graft on the anterior and posterior sides...so technically it is a stronger start to fusion than the posterior approach alone.