Just FYI but Single vs multiple rail is independent of the number of physical power connectors on a powersupply.
If the SC846 has enough independent physical connectors (generally using <=150W per 12V lead set is typical, this is enough for a GPU single 8 pin or two 6 pins assuming adapters) and you are within the total power limit then it is fine. To clarify this is per each set of wires that run all the way from the PSU housing, not multiple molex plugs spread across the same wires.
YMMV with shitty underpowered PSUs with thin gauge wires but typically supermicro uses good PSUs in its servers.
I have several high wattage single-rail powersupplies designed to handle 2/3/4 video cards without issue, I prefer single rail designs in my builds and most enthusiast models are these days.