I have two beers on line right now. Both have 5 ft of beer line. My PSI is set @ 11. The left tap (pale ale) pours fine but the right tap (pilsner) pours more foam. Thought it was the tap so I changed it last night and am still getting the same results. Guess I can try to switch the beer lines putting the pilsner on the left to see if that makes a difference. The shanks are 6 years old i that makes a differnence.
The shaft is solid and an unlikely culprit. I would look at other places air could be getting in the system from the keg tub gaskets to the tap. Also sometimes some stuck hops or beer stones in the line might cause this.
Have you cleaned them?
Are both taps using identical ID beer line?
Could your gasket between the shank and beer line coupler be off center creating flow irregularities?
As mentioned previously, could an air leak be the culprit? A simple spray of star san solution should bubble enough to reveal leaks while pouring.