The Duo Disk behaves exactly like two Disk II drives connected to a single Disk II controller card from a software point of view.
Do you want to copy a whole, unprotected, 5.25" disk? You can e.g. use COPYA from the Apple DOS 3.3 master disk, just say RUN COPYA at the "]" Applesoft BASIC prompt after booting from the DOS 3.3 master. If you use ...
You dealt with the problem of switching a bank under running
code by making sure that the next instruction address in the other bank had
the appropriate code to continue executing your program. This was set up by
setting the system to read from one bank but write to the other so that
your executing code could run from one bank but set up the other ...
On the Apple II, the values of address bits 0-4 and 7-9 during each part of a frame are independent of the graphics mode. Those eight bits are used to control the DRAM row address, and need to cycle through all possible bit patterns four times per frame (excluding bit 9, the remaining bits need to cycle through all bit patterns eight times per frame).
The Apple II 4116 DRAM RA0…RA6 row mapping is A7 A2 A0 A8 A3 A1 A12.
This is shown in a table in Figure 5.3 on page 5-6 of Understanding
the Apple II by Jim Sather.
This can be confirmed from the schematic in Figure 22-3 on p.112 of
the Apple II Reference Manual, which I have reproduced with annotations
The DRAM address line signals RA0…RA5 enter ...