Honestly it's not preatty clear to me what you intend, but if you want to schedule two tasks with two different timebases with only one clock, you could implement something like the following:
The clock1 fires every 1 second, and every 3 seconds it calls for the task that is executed every 3 seconds, while every 5 seconds it calls for the task executed every 5 seconds. (if this is what you need)
Please be aware that every 15 seconds (3*5) the two tasks will be executed both.