Or you can stagger the "Call of xxxxx".
Either software side, or with a filler spell.

I believe Tremor pulses every 3 seconds; the above post implies every 4 seconds which could be correct.
So with two Shammies, you'd like 1.5 seconds between drops.
With three Shammies, you'd like to drop a Tremor every second.
With four Shammies, you'd like to drop a Tremor every 3/4's of a second.

If you go with a software round-robin, time when you drop each set of totems.
And you can really benefit from staggering 3 or 4 Tremors.

If you go with filler spells, the GCD pretty much gets you totems at the 0.0 mark and 1.5 second mark (a tad more delay than 1.5, because of latency), so two staggered Tremors is probably the maximum benefit you'll see in practice.