You can rackmount a 2900, and there are enough bays for three half-height devices. The downside is that it's 5U.
If you're not so hung up on having the devices mounted internally, Quantum makes a nice 1U rackmount enclosure that has room for a pair of half-height devices. I don't believe they sell the enclosure separately, but I'm sure someone sells something similar. Take a look at CL1003-SST for an example of what I'm talking about. Coupled with a 1U server, this is probably going to be your most space efficient configuration with a traditional rack-mounted server.
OTOH, if you want to go REALLY space efficient, HP's (relatively) new blade enclosure has "tape backup blades" where each drive would take up a half-height blade slot.