I know that the +damage trinkets cannot be used together, but what is a macro (that will not show errors) that will use my first damage trinket, and later when the shared cooldown is over will use the second trinket?
Bass, I have a macro similar to that, and it will use the first one, then do nothing after the shared cooldown is over (I prefer it this way for me). The only difference is also the UIErrorsFrame stuff.
I believe he wants the second trinket to trigger 15 seconds (or whatever) after the first one.
You might try just setting up a cast sequence with a 2 minute reset.
This way you will use the first one, and the macro will queue up the second one. If you don't use the second one, when the first one is off cooldown it will switch back.
I believe the syntax is /castsequence reset=120 <trinket1>, <trinket2>