I will start with this. If you are a union shop, there is no such thing as a second man. The companies can't make you have one. Period, it's not in the contract. This caused a huge discussion in our area. Also if you are on a service route you have to take calls unless you have a gentlemans agreement with your coworkers.

That being said where I work we average 40 calls a weekend (Saturday and Sunday) during the summer months. Our schedule is set up with a two man system, but only for the man on call to use. The on call period is for two weeks. We have 6 men in total. you are the first man Monday, Wednesday, and Friday of week one and back up the other days. First man Tuesday, Thursday, Saturday and Sunday of the second week. Back up the other nights. The rotations overlap one week. So your back up first week is different the second week. Sounds complicated when written out but it's not. Only draw back is you kill two weekend. But down here it's worth it to have help when you need it. After your rotation, you're off for 4 weeks till it's your turn again. We used to have 10 in the rotation before cut backs. It was awesome back then you where only on call like twice a summer.
You decide when to call your back up out. I usually wait till I'm 6 or so behind. Then I dispatch the calls I want him to take for me. I usually try to give them the broke buttons and doors not closing calls. We try to get to the customers as soon as we can but sometimes you just can't help it here. Even with two guys are out.