Heavy leaning/steering forces?
 

When I got my Schwinn Twinn Sport 10-speed tandem, the original owner had replaced the captain's "drop" handlebars with "up" handlebars, he said so he would have more control. He still had the original bar, and gave it to me as part of the purchase. As I rode the bike with the "up" bars, it felt pretty normal to me.

Now I just put the original "drop" bars back on, and now I notice something odd as I ride, particularly with a stoker (my 9-year-old son) on the back. When we lean the bike slightly, there are significant forces that try to turn the front wheel in that direction. Happens equally right or left. It's a LOT more noticeable than on any single-seat bike with drop handlebars I've ever ridden. We made a few medium-slow corners last night, and I had to muscle it a little to maintain balance and make it go where I wanted.

I hadn't noticed this when the "up" handlebars were in place. Those kept my hands apart maybe 30", providing plenty of leverage. Now these "drop" handlebars have my hands maybe 10" apart (on center, meaning maybe 5" between the thumbs), obviously I have much less leverage. Or to look at it another way, the bike has a lot more leverage on ME! This is clearly why the original owner had replaced the drop handlebars with "up" handlebars.

All bikes, tandem or single, do this to some degree of course. That's what the forward curve in the front fork is for - it makes the bike naturally stable at low speeds, making it turn into any lean. At higher speeds, gyro forces on the wheel rims dominate and do the same thing. I've checked this fork, and there is no evidence of damage or bending - the fork arms are perfectly straight with the headset. And it's happened with either the old steel rim front wheel, or the new allow rim wheel, to about the same degree.

Do all tandems have noticeably heavier leaning-turning forces than single seat bikes, at low to medium speeds? Do you folks with drop bars on your tandems, notice this much compared to single-seat drop-bar bikes?

Generally speaking, yes... It's a simple function having more weight on the tandem, it's longer wheelbase, and the center of gravity being further away from the turning axis.

Add to that even a small-sized stoker who may intially be attempting to look around the captain to see the road ahead or leaning into our away from the turns instead of sitting in a neutral position and you end up with a bike that has a much higher steering task load than a single bike. The taller and heavier the stoker, the more pronouned the stoker-induced steering becomes.

I too suspect your stoker is encouraging this feel. I don't appreciably feel like the tandem steers differently from a single -- until the stoker moves, then it is like "whoa!". As a "classic" rider (read "old"), I spend a lot of time with my hands on the tops, just inches apart, elbows in. This is on a Co-Motion.

Also suggest you do not have a 'death grip' on the handlebars. Leave hands a bit loose and don't get tensed up. Perhaps taking the corner at speed slower than a crawl will also diminish that feeling.
Also stoker (child or adult) should not attempt or 'help' steer from the back!