The WB68 isn't cascadeable but what you can do is run all 4 lines from the AT9 into 4 wide-band power-passing splitters (5-3000 Mhz, or something like that) then run those 8 lines into the 8 in ports on 2 WB68's then run the 16 out ports from the WB68 into the IRD's.
I have the Solidsignal ones and they work fine splitting a Phase III feed to 2 Terk BMS-58's.
The problem I could see is I am not sure how far down the Ka feed is. It may go lower than 900 Mhz in which case they probably would not work. I know you can't diplex so there is a good chance it is putting some of the KA frequency below 900 Mhz.