I am unclear on some of the terminology. There are three kinds of SSD that are related, thsoe that go in a PCIe slot and stand vertically off the MB (Intel 750 has a variation for that), those that connect to a M.2 connector, and generally lay flat against the MB, and those that connect via a U.2 cable and are not physically mounted on the MB. These three types of connections are not interchangeable (well, without an adapter, for example there re several M.2 to U.2 adapters).
All take PCIe channels and may conflict with each other depending on the chipset and MB design, so you may find any given slot disabled if you put a SSD in another slot. You have to check the MB manual to tell which can be used together.