ORIGINAL: AviationNut
Why not cut the pieces at the same time? Put one sheet of monokote over another and cut them together - then they'd be exactly the same.
I'll take this a step further and say...
Apply one sheet of MK on top of the other with Trim Solvent (which I prefer to Windex) once it has set, you can then cut out the two pieces at the same time since they are already stuck together.
As for razor blades, use an X-Acto knife with a #11 blade