I contacted both Rosewill and SYBA's tech support, and here's what they said:
Rosewill: "We have many
models can be used in the same computer and will not cause the conflict. We are
wondering if you can confirm with us how many HDDs you want to use. Please also
let us know they are SATA or PATA HDDs. And then, we will be able to give you a
best recommendation."
SYBA: "Due to the same bios, you can not use two identical controllers into one
system. It will cause the conflict."
So, possible with Rosewill, but not possible with SYBA. Oddly enough, I noticed that some Rosewill and SYBA models share the same chipset