I only have 2x8 DDR4 RAM when doing test do I test with Both sticks in or just one?
For a dual-stick kit, test them both together first. Then, if there are errors, test one module at a time. Ultimately in that scenario, the problem could be either of the modules, both of them, or a motherboard RAM slot. But by process of elimination, you need to find out which it is.
Of course it takes longer to test two modules simultaneously than the time for a single module, but you'd have to test the second module anyway.
As for "coverage," the testing program's instructions should provide some guidelines. With HCI Memtest, 100% coverage (a single pass) may be insufficient. HCI says a 1,000% coverage test will be "thorough." For a new RAM kit configured to stock-spec settings, I'd be inclined to accept something between 300 and 500%. If you overclock the RAM, better shoot for the 1,000% test.
Just guessing, but I think certain statistical distributions would apply, such that the longer the test, the less likely another 100% pass will fail.