I built a series Speakon junction box, 2 inputs become one output. Cost me about $25 CDN at a local electronics supply store. They had little empty plastic boxes at the store, I drilled out a couple holes and mounted the jacks inside. I had them wired in series. I also made mine with speakon NL4, which is the standard on cabs and can handle bi-amping.
The thing works like a charm but I don't find myself using it because my 2 cabs are different resistance, I will get a power head and run them parallel if I ever need to run them both.
If you just want to buy my box off me PM me, mind you if your good with your hands it is easy to make yourself and cheaper, shipping over the 49th kills.
I can PM you the threads I had where people gave me wiring schematics for making a series speakon cable if you want too.
Best of luck with your rig!