Our local kids rec league pays 1.5 times the rate if you work a game alone. That's seems fair to me. When working alone, you don't do twice the work, but you do about 1.5 times the work.
There's a "competitive" kids league around here that only pays an extra $5 if you work a game alone. That sucks.
BTW - our local kids rec league is giving all the refs a raise this coming season. They asked me to come up with the new pay scale. Cool.