I'm assuming your canvas is 24x12 (or double the width of your templates, so both will fit)? Anyway, I'm going to use that size to explain how I put two 12x12 templates onto one canvas. First I do what you do and drag-and-drop the left-side template onto my canvas. They do place themselves in the middle of the canvas, but since all of the layers are selected, I just move the everything to the left until the background paper reaches the left edge of my canvas. Then I drag and drop the right-side template and move everything to the right. There shouldn't be any empty space in the middle of your canvas if you do this correctly.
There is a fancier way: after you drag/drop all of the template's layers onto the double canvas, right-click on any layer and select "Group From Layers" and just click OK (unless you care about the name; then change the name and then click OK.) Then, with the Move Tool selected, hit CTRL+A and at the top of your screen, choose the Align Left Edges (or Right, for the right template) option and everything will move to the left as a group--keeping the arrangement of the layers as-is.