For several years I fed GPS to a Gen 3 20/20 on my combine out of the factory GPS cable on a CIH 7240 combine.  I too was running a Pro 700 and 372 for RTK.  My issue was that I would harvest about 1,000x more acres on the 20/20 than on the Pro 700 and I was told it was a decimal place issue.  Two years ago I got the cable that goes from the second port on the 372 directly to the 20/20 and I have not had problems since.