Nick - it's a broad topic but will try to help. Using work queues is a must, so that you can have multiple instances working from the same queue. And environment locks enable you to control any 'one at a time' steps within the process logic. Apologies if you know this already but sometimes the basics are overlooked. https://portal.blueprism.com/documents/blue-prism-solution-design-overview
Using sub-processes is another idea, where if sub-process A has nothing to do, the parent process runs sub-process B.
Lastly, there is the option of making asynchronous calls via the command line, or in plainer English, creating a process that uses the command line to run processes on other machines. You're right in that this isn't well documented, mainly because it's not standard practice. When Active Queues comes to full fruition I think it will provide more of this kind of functionality 'out of the box' but for now you have to invent the wheel.
The fundamental idea is that process A uses the command line to run process B (on another machine if necessary). The command line outputs the session ID of B, which A can keep hold of to monitor B's progress.
Here at BP Professional Services we'll be working on ways to explain this better, hopefully with working examples, so watch this space.