Crisp created a PCB for the “DualCore SwinSID”. The two AVRs are piggybacked to simplify the connections. Both AVRs are plugged into sockets, so that it’s still possible to flash a new firmware (Forum Article).
The extension board can be plugged into the SID socket of a C64 and MBHP_SID module.
For the case that you missed it: link to SwinSID Review