Dual disk drives and the ready signal

[The Unready]

Hi,I recently bought a 3.5"-drive plus modified cable in a fancy little case with a 5V power supply and side switch, planning to use it alongside my FD-1 on an unmodded DDI-1. Unfortunately the FD-1 doesn't read anything as long as the other drive is connected, it tries and even moves the head (as long as it's drive B, if it's A it won't do anything but spin the disk) but always ends in a "read fail". The 3.5" drive works perfectly.
According to the guy who made the cable it's due to the ready signal being shorted and you can't have dual drives if the ready signal is always on. But why is that? Why do dual FD-1s work? As far as I understood it the FD-1 sends a ready signal as soon as the disk is spinning, so wouldn't the ready signal be there in any case at the moment the FDC tries to read from disk?Or could it be another problem after all, such as drive select?

[GUNHED]

A B-drive must be jumpered to DS1 and the Ready Signal must be working (Jumpers or solder jumper).

[The Unready]

Thank you for the answer! The 3.5" drive works fine as either A or B, so I guess the drive select lines must be jumpered/soldered together either in the drive or the cable mod (haven't opened it yet). When I get home I'll check if I can change it to B only.
On the other hand, wouldn't an FD-1 also be responding to both drive select lines as the DDI-1 handles drive select by only having one of the pins connected in each connector? So in theory it should always work as long as the drive is either jumpered to A  and plugged on the A connector, jumpered to B and plugged on the B connector or jumpered/soldered to both A and B, shouldn't it?

[The Unready]

Problem solved! Turns out the drive was already soldered to DS1, but the cable mod involved shorting Pin 12 to ground - thankfully with an extra wire which I could remove. Now it works perfectly fine.