Have you checked that you didn't swap ground and signal for either of the outputs? I probably would have used blue wire for both the grounds and red wire for both the signals to avoid this mistake.
You're tapping SID 1's output directly from the chip. Do you have any kind of circuitry between it and the output jack? You should ideally have a transistor circuitry like the one on the board, or just reuse the one on the C64 motherboard. Or at the very least a 1k resistor and 10 uF capacitor (example values) in series to protect the chip and remove the DC offset from the signal.

Hey Chipcrew,

I found my C64 with SID2SID board in it and wanted to use it again. The problem is that every time I use the chip select to use both SID's my C64 crashes. I already checked if the parts were soldered correctly (especially the transistor) and I think they are. Anyone care to help me out? I'll post some pictures below.

Thanks in advance!

http://imgur.com/nF8hnQA
http://imgur.com/FpWcIVt
http://imgur.com/qcd1xg6

Mod edit: fxed the image codes.