Well I was thinking about this a bit more, and I figured I'd better just try an experiment...
This IR filter was intended to block IR from a color CCD, but it seems to work great as a general purpose high-pass filter.
Here's the 632nm passing through the filter (I don't have a 635)
Now here's the 660 added in. I left them misaligned so you could see the two beams
The key was to get the angle right, which turned out to be about 30 degrees