@Crystal DundasElectric heat is definitely expensive and an issue with retaining renters. I have it in a house that's well insulated and it's been high for all tenants, even those who were careful with their heat. I would get prices for a gas heater and for a heat pump (you can also check out mini-split systems, which have the bonus of providing cooling, but since you have ducts already the other options may make more sense.)
You can absolutely have one unit electric the other gas, you just can't have one gas heater serve both units and bill to one tenant.
Another option which I've been using for my tenants while I procrastinate on selecting a from the choices above, is offer to raise their rent $25/mnth and give them $100 off per month Dec, Jan, and Feb. Or give them a bit more than that off and eat the difference.
Personally I've been subsidizing the heat costs a bit with a winter rent discount, my fault for procrastinating. Combine with putting up window plastic for them, door snakes or weather stripping, maybe a programmable thermostat, and the education tactics. Also, an easy oversight, sometimes the top part of the windows slip down so that they don't latch properly, a lot of heat can be lost there without realizing it.