Git – How to organize back-end database design and front-end application in software repository

Unless the project is really huge, I'd go for single repository with subdirectories for server and client and create a branch for each version. You can still have multiple copies of the repository in case you want access multiple versions at the same time.

By maintaining multiple repositories, you'd make transferring changes harder than necessary (rebase is easier than applying patches). In the (improbable) case there'll no changes to be applied to multiple versions, you still lose nothing...

Moreover, you can always switch to multiple repositories: Just clone the repo and remove the branches you don't want. Going the other way round is harder.

I'd go for multiple repos only if the server and client share nothing or if the code is really huge.

It's two different projects, so use two different repos. It's not hard to have post commit hooks in both. Having multiple projects in the same repo just makes it messier to work on one while you're able to see the other, which leads to multiple copies, which leads to confusion.... if you're using a hosting service like GitHub you can see the changes to all the repos under an Organization (and other services have similar idioms) on one page.