You just have to set the contents of $10113 and $10114 to the device numbers. Note that these are 20-bit C65-style addresses, so you have to do some jiggery-pokery to swap them if you are in C64 mode on a C65, since there is no 20-bit monitor in C64 mode.
You can work around this by using the OPEN command to command the device number changes, e.g.:
However, if you are on a MEGA65, life is much simpler: Long-press the RESTORE key to enter the Freeze Menu, and then press 8 to toggle the drive number of the first internal drive between 8 and 10, and 9 to toggle the second between 9 and 11, as you can see in these shots:
With this, you can easily have an external 1541 or similar on device 8, and do things like, say, load Sam's Journey:
Or better yet, see it in action:
Actually, one of the really great things about this post, is that all I did was a 5 minute fix to the Freeze Menu based on information that others had worked out, and the result was then tested by Anton. That is, this is more of a post by the community and for the community, than it is a usual "what has Paul been making lately" post. I hope that we see more and more of this kind of post, as the pre-series and then dev-kits and then retail versions of the MEGA65 get used more over time.