ewsb,
This is done quite frequently and usually where a heat pump just can't keep up, or shuts down due to cold temps. Actually the outer evaporator freezes up from the condensate water and quits. Most heat pump manufacturers offer a reverse defrost cycle that will thaw this frost, to allow operation down into colder weather conditions. AquaCal's models are called the Ice Breaker.
Fortunately this dual heater set up is done mostly for diehard swimmers in cold climate regions and is only for brief periods of the year so the cost is not tooooo bad.
You'll just need to install the gas heater after the heat pump and at a lower thermostat setting than the heat pump. When temps fall below the heat pump's ability to maintain it, the gas heater will kick on.
Hope this helps,