I've never come across ones that were set up with barbed ends for hose. I'm not saying they don't exist, but I've never come across them at Home Depot. I believe you can probably piece on together from stuff at the hardware store though. Get a ball valve (same diameter as your hose or a little larger) with threaded fittings and then two nylon barbed adaptors sized for your hose and the ball valve. This will probably run you about $9 tops. (About $5 for a valve and $2 for each barbed adaptor.)
As far as the issue, if the pump has't changed, all I can figure is that your single return constricted the flow to the point that it acted like a valve on your pump and wye-ing the return for dual opened up the restriction enough to releive the back pressure. That or something is clogging the overflow. . .
It is a violation of federal law to use this product in a manner inconsistent with its labelling.