Bicycle Mechanics - Compatibility of SRAM and Shimano cogs

I've got both a 13-30 (13-14-15-17-19-21-24-27-30) Shimano and 11-32 (11-12-14-16-18-21-24-28-32) SRAM cassette for my tandem. Are the cassettes compatible at the cog level such that I could create a 13-32 cassette (13-14-15-16-18-21-24-28-32)?

Most of the discussions I've come across talk about building custom cassettes from a single manufacturer's product line.

If the cogs are riveted together, you might not be able to re-rivet the cogs (no big deal though). If the cogs are assembled to a carrier, you very likely won't be able to mount the other manufacturer's cogs to the other's carrier (a deal breaker unless you drill new holes). If you are dealing with loose cogs, mix and match as much as you'd like. I doubt you'll notice any difference.

If the holes are spaced right, you might be able to do it. I like the cassettes that are held together with bolts, for cleaning purposes you can get them looking brand new!

I'm looking to do the same thing, but to create a 11-19 8speed for a road wheelset for my MTB and its 42/32/22 crank. Anyone know if the Nashbar branded cassettes will separate? I'm looking to start with their 12-21 and mix in an 11-12 from a SRAM 11-28.

if you use the shimano spacers after you pull apart the cassette by unscrewing the 1.5mm allen screws, then yes, you can mix them.
the sram spacers, depending on model, are designed to interlock with their cogs. shimano spacers are designed to be bolted together by a screw so they have no protrusions.
the cogs themselves are flat for both.