I have a setup today as follows:

an app on one linode (php-fpm + nginx)
a db on one linode (mysql)
a cache on one linode (redis)

Horizontally scaled. Works with no hickups. top-command show between 0.00 and 0.20 practically all the time. Fine.

I'm contemplating a new client (really a new app) using the same backend, same mysqlDB and cache as the original app.
I'm considering setting it on its own linode just like the first app i.e. php-fpm and nginx-instances running while calling the db and cache linodes over the private network.

And NOW, the question

Will it slow down the db considerably to get traffic from many different clients or is this a good way to go? Is there anything else I should be aware of when doing this?

The practicality of it will be that a user in one app exists in the other app and vice versa, just different "views" or "functions" if you will, as the two apps cater to two different audiences which sometimes overlap hence they share user base.

Thanks for any answer you may provide.

Generally speaking, MySQL doesn't give a damn where queries come from or what they're asking for. It's all sockets, as far as mysqld is concerned.

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Thanks. That pretty much sums up what I wanted to know.