Blender ocean-joining meshes evenly


I’ve looked on the boards and can’t find a way to join (after alt+d) my meshes. This is for an ocean sim with a wave modifier already attached. They don’t even have to be joined, preferably not, actually. In Cog’s tutorial Creating a Realistic Seascape (warning PDF) we’re told to evenly match our planes that we need to “…alter the size x value to negative.” I don’t know which x value to change or what to change it to. Can someone please help? I believe since this is technically an animated plane(s) that it belongs in this forum. Thanks for any help.

In 3D view press the ‘n’ key and you will see the transforms box. Highlight one of the planes you have created and then alter the scale x or scale y to negative as required to match the sides of the plane to the ones next to it.