A thank you to everyone who provided input to my previous post.
Using a combination of techniques/advice offered, I did the following:
First, a screwed both boards to a sacrificial piece of plywood (Thanks
Frank) with the boards spaced 17/32" apart (width of a 1/2" straight cutter
and then some).
I then attached the fence to my router and ran that against the outside of
one of the two planks (which is straight enough for these purposes) and made
progressively deeper cuts until reaching the depth limit of my 3" (usable)
straight cutting bit. After that, I moved the fence out slightly so the bit
was no longer cutting the first board but the second, in a mirror image of
the first (minor imperfections and all). After releasing the boards from
the plywood, they went together like silk, no light shining through the
Thanks again to everyone who provided suggestions.
I'll post a couple of pics in abpw if anyone is interested, just ask.