ssb-

Easy peasy. Once the *apparent* variables are worked out (hmmm... seems that two pendula of identical *length* seem to have the same period. Let me test this...)

One tests this timing hypothesis by one of two ways:

1) Start two swinging simultaneously but with different amplitude.

2) Start one swinging, let it decay for a time, then start another.

If you've hit upon the appropriate variable or combination of variables, they should remain synchronized.

Of course, that's for your hypothetical pendulum. Your chandelier or censer or whatever introduces friction, air resistance, possible added forces, etc. into the mix. One suspects that over short periods, a keen observer might suspect that synchronizing these things might appear simple, but as the time of observation increases, errors start appearing due various factors.