I don't know. At "my" carrier,737's are lit off #2 first then #1. If the APU is on MEL,then #1 is left running till 400Hz is plugged in. A lot of times,an engine will be changed on condition (failed boroscope inspection,performance,damage),and therefore each engine has a "life' of it's own. Just because a plane was delivered new,doesn't necessarily mean both engines will hit their TBO's at the same time,because of other issues over an engines lifespan. It still goes by hours on the wing,not by actual running time. I often suspect myself though,that certain items that get used on the ground will tend to be changed more often. Like,say,running the same fuel boost pumps for the APU (I always try to crossfeed off the other tank while I have a plane under "my possesion" while I'm working on it to mitigate extra wear and tear on the usual APU feed boost pump--a bit obsessive huh?). Actually though,I don't really see much correlation twixt components changed and their ostensibly greater..or less use.