Should you ever test multiple A/B tests at the same time?

Okay, so you have a website with lots of web pages on it. It has a header/nav and a footer that show up on every page, obviously. And you have a bunch of other unique individual pages.

You want to run some A/B tests on several different pages on the site. However, we also want to run some A/B tests on the navigation. The navigation that is on every page...

So, if you take some random tested page: you have different test variations of this page as well as different test variations of the global navigation on this page as well.

So how do you determine test winners when you have multiple tests at once? If a visitor is seeing Nav variation B and page variation B, how do you know which test is actually the one contributing to the "increase in conversions" (or whatever your success metric is)?

Alternatively, you could just split your traffic so that people are only in one test a time. It just seems like you are potentially limiting your audience (depending on how much traffic your site gets) if you have a lot of different tests on the site you want to run.

Or would another alternative be that whenever you need to test a global ui element (like a nav or footer) that you cease all other individual, one-off testing on the site? That seems like that would work but it really limits how much you can test during that time period.

What is the best approach to this problem? What is the best practice for deploying multiple tests on a site at the same time?