I only had single shaft type encoders in my component stock, so I converted these to dual-shaft types.
For rotary encoders, I used the ALPS type EC11B15242AF. These can be dismantled and with some work adapted to dual shaft type. There are also dual shaft types available from ALPS: EC11E0B2LB01
For details see ALPS rotary encoders data sheet

To convert the standard type rotary encoder to a dual shaft type, I placed two encoders in a structure above each other, and the bottom encoder is rotated via a 1.2mm steel wire, that goes through the shaft of the top encoder.

The encoder can be disassembled by carefully bending the cover clamps. I removed the top rotary encoder push switch, then drilled a 2mm hole in the back cover and metal cover. The knobs came from old equipment.

A 1.3mm hole needs to be drilled through the metal shaft of the top encoder. This can best be done by drilling into a rotating shaft. The bottom rotary encoder shaft needs a 1.2mm hole about 10mm deep for the 1.2mm steel wire. A hole with M2 thread is perpendicular on the vertical hole for the wire clamp screw. A 1mm plate aluminum frame holds both encoders.  


A 2.5mm carbon tube is glued on top of the 1.2mm steel wire, to fit the top knob. The two encoders are mounted in the frame. The steel wire goes through the top shaft, and is clamped into the bottom shaft via the M2 screw. To make the crew hold the 1.2mm wire, the wire is flattened at the screw position.

The push switch action of the bottom encoder can still be used, by allowing some 2mm extra length in the steel wire, so that the top knob can be moved up and down. 

Note that you need encoders with detent click, to keep the encoders from moving together when one shaft is turned.