The RT3WB080 has two SFF-8087 ports that consist of four SATA ports each. The RES2SV240 has 24 "ports" through six SFF-8087 connectors. The ports of the RES2SV240 are like network switches, with some limitations. You can connect one of the SFF-8087 ports from the RT3WB080 to the RES2SV240 and use the remaining 20 (five SFF-8087 ports) to connect to disk drives. Or you can connect the two SFF-8087 ports from the RT3WB080 to the RES2SV240 and use the remaining 16 (four SFF-8087 ports) to connect to disk drives.
The RT3WB080 individual ports then become waterfall type of connections. If you're pushing less than 6Gb (theoretical) from the RT3WB080 to the RES2SV240, you'll only use a single port. If you go over 6Gb, it'll fall over to additional ports to take up the slack. So in reality, using the two SFF-8087 ports from the RT3WB080 to the RES2SV240 is kind of overkill.
If you connect only one connection from RT3WB080 to the RES2SV240 you can use the other one to connect to to other HDDs, another backplane or expander. It gets more complicated when you do this kind of thing through. You shouldn't mix disks from separate expanders and backplanes in single arrays.
Thanks for the detailed answer. That is exactly what I was looking for and kind of what I though would happen. It would be nice if this information was brought out in the hardware documentation for the expander card. The documentation only shows one SFF8087 connected to the expander card so you are kind of left handing on what to do with the other SFF8087. It may be in the hardware documentation for the RT3WB080 and I will check.