Advice please

I have set up a slatted shelf along the length of my hut window and have placed 2 grow bag trays on the shelf both of which can be covered with large cloch covers, one will be sitting on a heat mat and used as a propagator, while the other will be used to grow on seedlings or cuttings, I also have a strip light which I can fit onto one of the covers, as both will be right beside the window which one should I fit the light onto?

I would probably put it on the one with the heat mat - to start with at any rate as would help the seedlings grow more stocky. When they are transferred to the other they would likely slow down a bit as it could be considerably cooler(?). I might then reconsider whether to move it with the first set of seedlings.
It would also depend how long the seedlings are left on heat once hatched.

Seeds don't need light but might like heat to germinate; seedlings need light but not as much heat. Depends on the overall temp of your shed and the type of seed.