If your disk controller card looks like the one below, then this adapter is for you.
The Dual 5.25 Adapter is an accessory for Apple’s standard Disk II Interface Card, bringing support for dual floppy drive emulation. Floppy Emu Model C can emulate two 5.25 inch Apple II floppy drives at the same time. That’s great for two-disk games and for reducing disk swapping. But dual 5.25 mode normally requires an Apple II computer or disk controller card with a 19-pin D-SUB DB-19 connector. That leaves out Apple II+ and IIe owners who have the older Disk II Interface Card with two separate 10 x 2 pin rectangular disk connectors.
This adapter makes it possible to use dual 5.25 mode with the Disk II Interface Card. Why all the fuss about DB-19 versus 10 x 2 rectangular connectors? The DB-19 has two independent logic signals for enabling two separate disk drives, but the rectangular connector only has a single enable signal. This adapter taps into the enable signals from both rectangular connectors on the Disk II Interface Card, and combines them.
The Dual 5.25 Adapter isn’t required, but it makes everything more convenient for a Floppy Emu paired with a Disk II Interface Card. Without the adapter, the Disk II Interface Card works for Floppy Emu’s single 5.25 inch floppy emulation mode, but dual 5.25 mode is not supported.
Please see the Floppy Emu instruction manual for full compatibility details for its Dual 5.25 Inch Floppy emulation mode.