The easiest way would be to install commercial software such as Parallels Virtuozzo on a clean server:
http://www.parallels.com/products/virtuozzo/
Secure the server thoroughly. Install any required packages on the server.
Create two "hybrid" VMs - that is, split the processors, memory, and etc. using Virtuozzo's built in options.
Install cPanel on one VM and DirectAdmin on the other. It should work without any tweaking.
Of course, there are a number of disadvantages to doing this:
1. Increased load on the server from running two control panels.
2. Increased memory footprint from doubling services.
3. Increased strain on the hard drives.
4. Limited software configuration due to being in a virtual machine.
5. The cost of three licenses: DirectAdmin, cPanel, Virtuozzo.