To add to @JimHowell1970's excellent answer: I designed to top two rows to be completely usable as a synth on its own - VCOs into VCF into VCAs with lots of modulation capabilities. The Grids can be used as the clock source and the Pico DSP can be considered its "output module". I have a Twisted Electrons Crazy8 laying around that I can use as the sequencer.
When paired with the lower half, the Clock 'O Pawn becomes the clock source and the Manther Growl and the A-110-2 become the main sound sources, or an added bassline to the sound of Plaits and tELHARMONIC, or just a mayhem of distorted sounds alltogether 😁
Another interesting tidbit about combining 2 Rackbrutes is that, by default, the lower half mounts "upside down", as can be seen in this video by BoBeats. This means the power supply of the lower Rackbrute will be in the bottom right, which disturbed me. However, by unscrewing the wooden side panels it's possible to "flip" the internal assembly so that the hinge can be mounted on the other side, making the two Rackbrutes in the same orientation again. Definitely something you'll want to do when getting your second Rackbrute!