Hot water in my downstairs shower does not work when upstairs shower (hot water) is on. I have two water heaters in my garage, Hot water runs fine both up and downstairs but not at the same time.

Do you have a recirculation line?

The title of your post says that the downstairs shower has hot water only when the upstairs shower is in use. In your post you said that the downstairs does not not work when the upstairs shower is in use. Which is it?