I used both forks. This isn't a fork adapter that hangs off the bucket, this is a quick detach fork capable of about 2k. I welded two pieces of 3 1/2 foot long rectangular tubing together. Then I laid them centered over an I-beam and welded them up each side. I have not tested how much weight it will lift, but it isn't coming off. I slide both forks together and just drive into the rectangular tubing. My goal was to be able to set trusses so I do not need much lift capacity. I know the weight of the I-beam is heavy which reduces how much I can lift, but it was a free I beam given to me by a friend so I used it. It was 14 feet long so on the New Holland 45 with I-beam mounted on the forks I can lift 23 ½ feet tall. I have a 758c backhoe mounted on my TC-45 which probably gives a good amount of counter balance.
I have seen somewhere on here a couple of good ideas like you are suggesting. One guy posted pictures on here about using a basement jack slid onto a single fork for setting trusses. He used one fork, and it was a round basement jack so it just slid over the top of the fork. If it were small loads it would probably be OK. I would be concerned about bending something if both lift arms were not used together on a heavy load, which is why I build mine like I did. I wonder if you could piggy back off the other guys idea and use 2 basement jacks welded or chained together for equal pressure on each lift arm but a light pole which allows more weight capacity and less counterbalance needed. Paul