So, I have 2 zones, each has a thermostat that has a "Circ" mode which is described as a 30% "duty cycle" on the circulation fan.
I'm pretty sure that the fan activates the correct zones, also.

But having 2 thermostats, would that mean potentially the system could run the fan 60% of the time because each thermostat might call for its 30% at a different time, or do they "work together" somehow through the zoning panel?

 I'm guessing the first, and I'm wondering if it's possible to make the 30% be simultaneous by having one thermo control both zones in fan mode (but not in cooling/heating) maybe using logic circuits and relays somehow?

That could be a problem. I don't think the stats are smart enough that they would talk to each other. Similarly, if one stat is calling for 'cooling' and the other is calling for 'heat, then the system would probably wig out. I'd probably just set one to 'cir' and leave the other on 'auto'.

One pro suggested to me last year that Circ not be used due to extra on/off cycles causing unnecessary wear on the ECM blower motor. If I understand this correctly, power consumption is very low with ECM blowers and it may be better to just leave the blower on all the time if Auto is not used. Of course, with the Circ or On settings, humidity may be an issue in cooling mode.

I'd avoid circ mode altogether in a zoned system

On mode is OK in winter but not summer in most places where humidity is a concern in summer

Zone controls don't tend to "wig out".
If one zone calls for heating and one for cooling, zone system will attempt to satisfy #1 priority first. Priority is some times determined by zone (i.e. system satisfies zones in order 1st zone 1, then #2 etc.).
Sometimes determined by task (heating takes priority over cooling, DHW over heating etc.).
Priority may be hardwired into zone control or set up with dip switches. Some systems have to be manually changed from heat to cool.
Regardless of what system you have, circulation will likely work in harmony like heating or cooling. IOWs if only one zone is calling that zone will get circ. If other zone calls while first one still has circ on, then both zones will be open to the fan.
I agree circ in the summer ( if A/C is counted on for dehumidification) is ill advised.
joe

My concern about circ mode is that in a zoned system it will cause a large number of extra blower starts. While I'm a great fan of ECM blowers, I don't yet know whether an extra 20 or so starts per day will impact durability. I know that ECM blowers are very expensive to replace, hence my ambivalence.

Summer anywhere outdoor dewpoint exceeds 60*F, leave it in auto. Otherwise suit yourself - ECM blowers are most excellent at moving moderate airflow using very low power, good at breaking up stratification and keeping closed rooms fresh.