say if you have the old /use 13 and /use 14 in your macro, and you have trinket #1 is +1000 spell power, and trinket #2 is +1299 spell power.
/use 13 will fire, then it triggers the 30 second or 1 minute cooldown to use trinkets (dont remember exactly what it is). Even though the actual /use 13 trinket is on cooldown for 3 minutes.
After the 30 seconds or minute cooldown is done it will fire /use 14, until the actual trinkets cooldown reset the macro fires without the use of trinkets.

OR...
/use 13 is + spell power and /use 14 is + haste or any other stat besides spell power (for the msot part).
when your macro fires it will fire both trinkets at the same time, and then fire them again when their 3 minute cooldown is up or whenever the actual trinket cooldown is up.

As long as the buffs are stackable the macros will fire at the same time. If they dont stack they will fire one at a time when avaliable.