SSD and Superdrive on Nvidia MCP79 SATA controller
There are two independent SATA channels available. Each channel will negotiate highest possible speed based on client capabilities. It will not divide 3Gbps between two clients.

And as a side answer to your question: in case you don't use Superdrive look for SATA to SATA 9.5mm optical bay caddy. This way you'll be able to put SSD in place of Superdrive and have plenty of storage on platters.

Your Macbook even accept 12.5mm thick HDDs in HDD chamber.