Can you use 1 ecobee thermostat to control 2 zone valves for heating?
unfortunately the ecobee is not set up to do that.  Each thermostat only controls one heating zone, regardless of the quantity of room sensors.  The way the sensors are used is fairly flexible.  They can be included in an average with the t-stat and other sensors to trigger heat demand, and they can individually be included/excluded in different "comfort settings".  i.e. you can exclude the living room sensor for the "sleep" comfort setting when theoretically nobody will be in the living room.  You can also set them up as occupancy sensors for them to be included only when motion is sensed.

But yes, it would be pretty cool if each sensor (up to a limit) could be used as its own zone.  That would, however, require additional wires from the thermostat.  And the number would be limited- there's only 11 terminals available on today's ecobees, and lot of them will be used already, depending on what you already need hooked up.  So if your scenario had unused ecobee terminals, and if the ecobee programming was set up for it (it is not, at least today, and I don't have anything that suggests to me that that will ever change), then THEORETICALLY you could attach a separate pair of wires to the "zone 2" connections on your Taco ZVC-403 to control the second zone.  Your original ecobee/zone should have already occupied zone 1 in the ZVC403.

This question is a couple months old, so you've probably already figured this out, but to inject that ZVC into your system, all thermostats go into the ZVC, and the ZVC's "isolated end switch" replaces the thermostat wires into your boiler.  There's more to it than that, obviously, but Taco does have a bunch of helpful youtube videos out there to walk you through it.  Good luck!