Yes, there are potential problems with doing this directly. Yes, if done properly, you can have two items control a single circulator. The safest way to do this is probably with an additional controller. It would take the inputs from the existing devices, and control the circulator. If both relays were on the same leg of the power, it might work. If someone moved a breaker, you'd mess things up the first time it came on. www.tacohvac.com among others, make controllers designed for combining things like this.