Re-use circuit breaker handle tie?
For this to work, the two single breakers must pull power off each bus leg to give you 240VAC. If they pull off the same leg, you only get 120VAC (parallel feed). Check to make sure they use alternate bus connections when they're side-by-side.

If the breaker handles have a hole, I've seen a cut nail used to link them. They must be linked so both legs shut off for the obvious reason.