Fairly often, we will get someone come into our store with two rackets with broken strings. The question they usually ask is,
"why did the strings break on both in the same match?"
The reason is pretty simple.
They were equally used, therefore, equally worn.
Think of it like this. You have two identical trees in your front yard and you are going to chop them down. You decide to hit one tree with an ax, then the other. You keep taking turns, hitting one, then the other. When the first tree goes over, the other should be very close to falling as well.
It is the same thing with your strings on multiple rackets. If you are alternating them perfectly, when a string goes on one, they are going to be very close to breaking on the other.
So, what should you do?
There are a few ways to avoid this problem. The first and most obvious is to use one until it breaks and save the other. This will greatly increase the chance of getting through a tournament without the other breaking or just until you can get the other restrung.
There are some players who don't like saving a racket. For those, a good way to do it would be to use one racket more than the other (think 70% of the time for one, 30% for the other). This will keep enough difference in wear that you should be able to get through a match or two with some confidence.
One thing to keep in mind is that I am talking about string breakage through wear and use. Mis-hit breaks and breaks caused by things other than wear are unpredictable and the above won't help.
The biggest thing to consider is that you want to be able to get through the rest of your match or tournament with your backup racket without unnecessary fear of breakage. By making sure your backup has significantly less wear will help get you through with much less worry.
