You cannot perform unicast stream splitting on a single WAAS, it is designed for edge splitting over a WAN link with a box at either end of the pipe. WAAS requires another WAAS WAE upstream to perform optimization between the two, which will allows the edge box to set up stream splitting.
For a DC only solution, I would look at ACNS running on the WAEs, either single box or load balanced WAEs via WCCP or L4-7 Switch. This will allow you to perform unicast splitting, Unicast-Multicast conversion or several other options.
Hope that helps,
Dan