I have a lot of experience with hand pain. I also have a lot of bikes. I have one of them dialed in and am in the process of working on the others. There has been good advice in the thread but much of it conflicting. I hope to offer a comprehensive solution in one post.

I haven't seen anywhere where you mention your height or reach and what size the bike is. My Raleigh Tandem is slightly too big for me. I put Ergon grips on it but they did nothing. Not saying that they aren't a good idea but they are not magic. Have also bought a Delta Threadless Stem Extender. I see where you have perhaps another 1/2" (?) of stem rise left on yours. You should use it. At minimum your handlebars should be an inch higher than your seat. Better would be 2". Ideal would be 4". That puts you in Dutch Workbike territory and you present maximum frontal area to the wind. Not good. But comfortable. Very. On my other tandem I have the handlebars 2" above seat height and I have the seat back as far as possible. That bike is somewhat undersized for me but it is a cruiser type tandem and the handlebars have an insane amount of rise. I am using no stem extension whatsoever. I could if I wanted get the handlebars 4" or more above seat height but 2" is fine and gives me a lot more forward lean than the average cruiser type likes but I am not a cruiser type.

I am also more or less a KOPS (knee over pedal spindle) advocate. In theory moving the seat fore and aft is soley to put your knee over the pedal spindle. On your type of bike your knee is likely behind the pedal spindle by some amount with the seat centered over the seatpost. I wouldn't want a plumbline dropped from the front of my kneecap to fall much more than an inch behind the pedal spindle with the pedal at the 3 o'clock (forward horizontal) position. Presumably the ball of your foot is also centered over the pedal spindle.

I really think on your bike the problem is handlebar height. For $20 a stem extender is a cheap solution. There are also shorter stems although yours is pretty short. Still, because your headtube is tilted ~73* as you extend it upward you will also be moving backward by some amount. You will get a shorter stem by default as you raise your handlebars with the stem extender. You may have to get longer brake and shifter cables but likely not. Keep us posted.

H