You said they work well together, but I am not sure if this means they can be used at the same time?

work well together doesn't mean using it together. Dr. Pickart have mention that GHK-cu break down easily. so it will be good to use exfol product and GHK-cu at different end of the day.

Thank you

There was a report in Dermatology Times a few years ago that said GHK-copper and retinoic acid were used together and the overall result was better than either alone.

So the best way would be to first apply the GHK-copper, then the retinoic acid cream. The retinoic acid cream will push more GHK-copper into the skin.