Can you combine hydroquinone with Retin-A or do you have to apply them separately?

I have a dark scar on my leg which I want to try to get rid of or lighten as much as possible. I have both Retin A .1% and hydroquinone 4%. How to I apply these two on my scar?. Do I combine them and then put it on my scar or do I have to put them separately and if so when/how? And which one do I put on first?