Hi Lisa,
Considering the very lightweight and relatively open weave of the material the inner sleeping compartments are made from, it will be difficult to provide a comfortable 17 to 21C inside on a cold night even with a powerful heater.
A good sleeping bag is essential. (4 season for 0C conditions), but also an insulating mat under the bag helps to reduce heat loss where the fabric has been compressed by body weight. Such mats are sold by camping shops.
A lot of heat is lost from the head, so a woolly hat or a sleeping bag with a hood will be the most effective.
A hat water bottle can be a real comfort to warm the bag before getting into it.
If you are really intent (please excuse the pun) on warming the whole sleeping compartment, then it would be wise to improve the insulation of the tent, otherwise you will basically be heating the whole awning, and the night sky.
I have never actually tried this, but well prepared hikers, will use many thin layers of clothing rather than one thick one, for comfort and heat retention. So If you have two sleeping compartments put one inside the other to double the effective wall thickness.
Alternatively, erect a small hiking tent inside the awning. These have better heat retention, because the fabric is often heat reflective, and a closer weave, that reduced thermal convection through the fabric. The down side of these tents is the fact that moisture will not pass through the fabric, so condensation on the inside can become a problem after two people sleeping all night and breathing out lots of water vapour. A canvas tent is more comfortable as the condensation will actually wick through the outer canvas by capillary action and evaporate from the outside.
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