That is a very low-end crankset. On a regular bike replacing the whole crankset with another low-end crankset would be a simple (and inexpensive) process.

But on a tandem the drive train is much more complicated and your options are much more limited. You really need to talk to the mechanics at your LBS about the options you have. It can be done -but it probably won't be cheap as the options are limited.

Those unicycle crank arms are totally not the same thing and can't be used here. You will need cranks with "spiders" on them that mount separate chainrings -if you have a used bicycle cooperative you might have a better chance of finding something that you need although the knowledge to know WHAT you need might not be easily available. You don't want to mess with changing the gearing so the chainrings you find will need to be the same numbers of teeth unless the person who is helping you knows what they are doing with tandems.

Then there is the issue with chainlines (the distance the chainrings are from the frame tube) The chainlines of the tandems might be much different than what you can find off of regular bicycles. This isn't something that can be easily explained or helped online with advice to walk you through this. You need the active involvement of an experienced bike mechanic who knows tandems as you chose these parts to convert to shorter cranks.