Select both faces, in the channel editor you should see (click to show), click that, you should see x,y,z colomns, select either the x or z axis numbers for both faces in these colomns, (hold down shift to select more than one), they should turn bold, hold control, and middle mouse drag left/right or up/down.

Holding down control will make it so your objects only scale on the axis NOT in bold. This is a little tricky I know, but it does work.