So you already have 4wire cabling to your SRX715. That's good. If you like to biamp the 715s, keep it that way. There are numerous advantages in biamping. Many of them will work out well if you bring the 715s to high power, which I guess isn't something you do everyday, since two SRX718 are enough low end for your application.
To address your problem: Why don't you use a 4wire cable for the subs too? Channel A of your sub amp will be on 1+/1- of the NL4 speakon, channel B will be on 2+/2-. All you have to do is to plug this 4wire cable in the first SRX718, put another short 4wire cable between the two SRX718 and set the switch below the speakon connectors on sub 1 to 1+/- and the switch on sub 2 to 2+/-. Every subwoofer no connects to his own channel of your amplifier and you have to run only one 4wire cable to the subs.
Another tip: If you'd like to simplify your setup, get rid of the XLS2000 and the DriveRack+ and get another Xti6002. Now use one Xti6002 per SRX715 (ch A for the 15 and ch B for HF or the other way around) and use the internal DSP (I guess there are fitting setups available). A further advantage to this would be that you could drive both SRX715 biamped mono from one amp without reconfiguring anything if one of the Xtis dies.