Yeah I'm sure it probably wasnt grounded properly when I tried the y cable splitter. I tried the cheapiest cables I could find. I wasnt suggesting anyone go buy a $100 pedal to split the signal but I know a few people might already have the boss tuner pedal. Just putting it out there as an easy option if you already have it.
I just hooked upped the Boss TU-2 tuner pedal. I used one output for rocksmith and the other one (bypass) for my guitar amp. It seemed to work pretty good. So if you have a boss tuner pedal with two outputs that could be an option. I can't believe I didn't think of this 6 months ago instead of buying a y splitter for my guitar cable that didn't work too well. Glad I read this thread, Thanks guys!
I tried this about 6 months ago with the y splitter cable on the guitar and I also ran one on my computers sound and ran it into the line in on my amp. I had a lot of problems and stopped. After I was reading this thread I was wondering has anyone just tried to use the boss tuner pedal (Boss TU-3) for the splitting the signal? I will try shortly and report back on how it works?