RainMachine can support two valves per zone if needed, but only if their combined amperage requirements do not exceed 750mA. If they do, you could encounter an overcurrent. Exercise caution and be sure you understand the electrical requirements of your setup.
Wiring more than one valve per zone terminal should only be done no further zones can be connected, and is not a course of action we recommend. That noted, it can be done if the above requirements are met.