Rule #1: The stoker is never wrong.
Srsly, it has many advantages over two solo bikes, especially if the two people involved have unequal strength.No need to toil to keep up, no need to dawdle waiting for your partner to catch up. The main thing is communication between you, especially when changing pace, gear or direction: calling out turns, braking etc is vital. Clipless (eg SPD

pedals are a great help: when Mrs eck and I first got our tandem, it had flat pedals and it meant that when I changed gear or freewheeled, her feet would fly off the pedals. Staying clipped in is a great help.
It's also essential that the stoker trusts the pilot and doesn't try to steer from the back. I'm lucky that Mrs eck is much more petite than I am, which makes driving the thing a lot easier. Ok, so they are harder work uphill, but can bowl along happily on the flat, and are brilliant on descents.
If your relationship is good, you'll love it. If it's not, don't even think about it. HTH