from C64scene homepage:

Tomi Malinen is currently developing a dual SID board, although there are other solutions available for your C64 this one is interesting because of the ability to use either 6581 or 8580 as SID #2 no matter the board revision.

This means that you can play games with 6581 sound from the first channel and 8580 sound from the second.

Other features of the dual SID board are:

• Small board, only about 40mm x 50mm.
• SID #2 address jumper selectable between D400, D420, D500, DE00, DF00.
• MSSIAH-compatibility at DE00.
• Full game compatibility with double mono setting at D400.

Installation requires soldering, two wires attach to the cartridge port and two have to be soldered to the Kernal IC. When the SID #2 is mapped to D400, D420, D500 the wires to the cartridge port are not used.

Photo of both protypes installed:

source: c64scene.com lemon64.com

I’m the creator of the SID 6581<>8580 Switcher (here the original project).

I’m Very happy to see my project on the net. Arkanoid (a user of Lemon64 Forum) has done a very nice PCB with some fixes. Very nice job.

This is not a SId2Sid Clone.

source: lemon64.com

This is a project for switch from SID 6581 to SID 8580 and vice versa. This is not a SId2Sid Clone. This is a Switcher ;-D

source: 6581-Wiki