You would have to seal the tent and just run a duct from outside tent, through light and then outside of tent to vent heat from the light but still maintain a sealed environment.
There are other options other than CO2 tanks & controllers but I have no experience with any CO2 supplementation. I think for the most consistent max CO2 ppm the tanks and controllers are the way to go. The other options sometimes only raise ppm a certain amount and can never reach optimum level or sustain it for an extended period of time.
As for the tent option what would suit your needs better, one large area or two smaller ones?
If you buy a 4x8 then both lights will be in that and the 4x4 you have now will be doing nothing. Is the fan you currently have big enough for two 600s in a 4x8?
If you buy another 4x4 then you'll need to get a second fan unless it's big enough to cool both tents and you vent both tents using a Y connector and the fan outside pulling from both tents.