Sheldon Brown (of course) has a good page on riding tandem. He doesn't get into details, but since the stoker's main job is to keep upright it doesn't demand a lot of bike handling skill. Riding as stoker for the first time is similar to being passenger on a motorbike for the first time; the main thing to learn is not to lean or shift your weight. All the bike-handling skill comes from the captain, and as the captain it's a good idea to become familiar with the handling of the tandem before you try riding with a stoker. (Brown's page covers all this.)
In my experience the trickiest part of riding stoker is getting mounted; holding steady balance with both feet on the pedals as the captain mounts. You might try having a pole or something similar for the stoker to put a hand on to help their balance.
So the answer is no, the stoker doesn't really need to learn how to ride, in the sense of knowing bike-handling skills. That said, the stoker does need to learn how to sit on the saddle without throwing the balance of the bike, and also to learn to turn the pedals at the same time - and in the same cadence - as the captain. Again, from experience, synchronized pedalling is one of the hardest parts of riding tandem, especially if you're an experienced cyclist who tends to spin continuously.
Imagine: you are with two people, you have one bike, and you want to go to a friend’s birthday party. What would you do? You could of course leave the bike and go walking, go by car or take a tram. But you can also be inspired by these Amsterdammers and go together on one bike.
We show you five different ways to share a bike (also called doubling). No special seats or cargo bikes needed!
1. Sit on the back carrier (one leg at each side).
Sitting on the back carrier is the most common way to cycle together. Men normally sit with one leg at each side
2. Sit on the back carrier (two legs to the same side)
This is the version that women like best.
3. Sit on the front carrier
A very popular way amongst Amsterdams youth. (Don’t try this with a heavy person).
4. Stand on the rear carrier
For a good view. Like this son on the back of his fathers bike.
5. Sit on the rear carrier facing backwards
Not a very clear picture. They went too fast and I don’t see this way very often. We actually do not know why you would do this. Maybe when the person cycling doesn’t smell too good, or you prefer looking at the streets instead of looking at a back?
There are a lot of other ways to cycle together on one bike (sit on the crossbar, on the handlebars, or on the saddle). Take a look at more pics in this Cycle Chic Republic post.
Now, after being inspired by these cyclists from Amsterdam would you take a bike together?