Suppose I have two timers in a game running at the same time. The first is set for updates every 1 sec, and the second every 10 secs. What happens when the two timers coincide? Suppose both timers update the same database.
this very much depends on what the timed tasks should do. Is it (at least in theory) possible for the two tasks to run in parallel? For your example with databases there are a lot of things which could make a difference. Is it read or write access? The same data? Same database? Are you using transactions? And so on...
Moreover it may also depend on your hardware. Even if it is possible to run two tasks in parallel in theory and your application is programmed with multithreading it's not possible to execute two threads at the same time on a single processor/core machine. They may appear to a human as if they would run in at the same time, but they don't. Even with multicore processors it may depend on lots of other things if the tasks can really run in parallel.
Another important point that comes to mind is the required accuracy. Is it enough if the said tasks execute within the same minute? Second? Millisecond?
As you see it's almost impossible to give a general answer to your questions which would be valid for all usage scenarios. You have to know some more specific requirements...
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