Instead of 5 port jmb585 use 6 port asm1166. Radxa recently added such card to their portfolio, I’m waiting for some details from @melo about it.
For best way to get 10Gbit (double!) and place for two m.2 sata cards checkout this link.
Also search for @seni bifurcation card here on forum.
This one probably will not work, its 16x to two 8x. Probably sata power is for one slot, the other one will get power from pci slot. There is only 5v/3v m.2 on rock 5B. This makes such setup big and bulky with ugly cabling.
You should be fine with Seni bifurcation board, one with sata ports and other with 10Gbit adapter.
