Remember that racquets are only tools to get a job done. They don't need to be babied and can take abuse. Once you accept that you'll relax quite a bit and have more fun!
Change your strings at least twice a year (absolute minimum). If you play with poly I'm amazed you were able to play with it for so long without your arm getting in trouble.
As far as playing with one and the other as a backup. If they are the same racquet then rotate them every set or even every two games (on changeovers). No reason to have a backup just use the two sticks out.
If they are not the same model then it's not smart to switch sticks every two games. I'd get two identical sticks.
Sucking at the baseline, S&V, D2 '88-90