If you want to make 2 x 2s into 4s dont bother with a Y-lead.
Just use a connector in the + of one battery and the - of the other (see diagram). Then make a single bridge connector for the other + and -.
I use this method on various models and never had a problem. you can make the leads just the right lenght to keep it tidy.
You could just run it straight from the ESC (like the diagram) and then you would only need the bridge cable.
http://www.rctech.net/forum/attachme...iring-lipo.jpg