Perhaps the best known way to rack mount a couple of individual 500 series modules in a single rack space is the A-Designs 500HR - a dual slot 500-series power supply and frame.
It has balanced XLR input and output for 2 mono channels, and can supply +48V phantom power. In addition to being the perfect companion for A Designs Audio 500 series modules, the 500HR is compatible with most VPR Alliance modules, and is the most affordable solution for any pair of 500 series processors. The 500HR can be used with all sorts of different modules to give you all sorts of customizable functionality. Loaded with two preamps it becomes a dual-channel mic pre; with two A-Designs EM-PEQs, it's a dual-channel, Pultec-style equalizer; and with one preamp and one EQ it is a complete pre/post EQ recording channel. Plus, using the preamp and EM-PEQ configuration, you can bypass the preamp on your guitar/bass/keyboard amplifier to give you a totally different amp for live performance. Preamp and compressor, compressor and delay and EQ, two different kinds of EQ, one tube and one solid- state compressor - the possibilities are literally endless.