blokhead,
GrandFather is right on one account, you only have the specified number of timers in each amount. While you can re-use the same timer, you are restricted to whatever state it is in when you use it.

For instance, if you have the 7 minute timer in the root thread split at 6/1, you don't have another 7 minute timer available to use.

GrandFather's second argument is incorrect though. You do not need to run the timers continously once they have been started - they can be turned on their sides for later use. This doesn't matter at all when you only have 2 timers but it would make a difference with more timers.

This of course is just a fun puzzle so people can solve it either way. Your solution with an invalid assumption is fine too. That link you sent me last night is up again so I am checking it out as well as reading your solution.