1. Dual channel boards should always be equipped with 1 stick for each of those 2 channels. I don't know what is going on of late; this is a question that was pretty much never seen on tech forums and now for whatever reasons it's become common.
2. RAM is guaranteed, usually for life for the product that comes in the package. So if you buy 1 stick and add another, you have o warranty which would allow you to return one or both.
3. RAM is tested at the factory ... all the ones in the same package anyway. RAM yields off the production line decrease with increased speed and lower latency. Getting 2 to work together shrinks that yield more.
4. If you match make, model, size, speed, CAS, module OEM for a 2nd stick, you have a "chance" of two separately purchased stocks working. So if you have 1 stick od DDR4-2133 CAS 16 your chances of a 2nd stick working are **relatively** much higher than DDR4-3200 CAS 15
5. In that case you might want to take a chance and buy a 2nd 8GB stick. Up at 3200, I wouldn't bother. Id sell the current stick and replace it with 2 x 8GB (recommended) set or 2 x 4GB is budget is strained.