Humphrey also only has one. We had a scan done recently to attempt to detect where the other one was, - my vet said it might be possible to remove it without him being fully castrated, which would have been a good option, Humphrey's however appears to still be in the stomach cavity and is barely visible, so removing it would be a bigger deal altogether. In the vet's opinion, they only really start to cause problems, if at all, after the age of about 5 so he has recommended yearly scans for the time being, just to monitor what's happening.
The scan itself was absolutely no problem, (Humphrey found it boring and fell asleep!) isn't invasive and cost us €30,00 so, in our case at least, seems to be the best way forward. I too have read so much conflicting information, depending on whether its from UK, US or Germany but I get the impression that the general trend is the belief that its not as high a risk as previously thought. I hope I'm making the right decision but would also be interested to hear if anyone learns of any new research, it does seem to be particularly common in cockers I think.