The standard Y-Adapter doubles the input L1/L2 to two outputs of each L1/L2. So the channels will behave as parallel wired.
The cross over adapter has one input L1/L2, one output L1/L2 and one L2/L1. If it is not available you can rewiring yourself if you can solder and do not have two left hands.
There are some pictures of it in a german forum but I am not shure if you can understand it, but at least you will see the picture.
http://www.aqua-it.de/phpBB2/viewtopic.php?p=2087#2087