A good way to keep two tents warm is utilizing some timers and fans.I had already ordered the 600w hps light when I saw this. We need HID lighting during the winter in at least 2 tents to keep it warm enough in our basement to grow good meds. We have a vent from our furnace and small heaters but it being a very old unfinished basement it has a hard time holding heat in the winter. That would be a perfect light for spring and summer though. If it's still available when our cashflow is better we'll try and get that light from ya. I appreciate the gesture too.
Have two tents running opposite 12 and 12 light schedules, one is always on while the other is off. Can use a automatic relay for the switch for simplicity, or various timers to do it. Can then double up useage of one circuit.
Use the warm exhaust from the lights on tent to vent into the dark tent, and a reverse of that when the lights change happens. Make it automatic, or just do manually.
Another similar option that I was using for my first winter indoors was to have my veg on 24 hr light, and vent its warm air into the flower room nonstop for the flower rooms intake.