Time's moved on a lot since this question was originally asked. Now, e-bikes are more affordable (okay still expensive, but better)
A modern ebike with a low assist level can be used to start with, perhaps ramping up the assist level as the ride progresses.
Fast rider can use this as motivation, or perhaps even draught behind the ebike (purely for practice ...
There are some good answers here, but I don't see my favourite, which is for the stronger rider to set a demonic pace at the front, lashing the other with a constant stream of imprecations and exhortations, turning round with a furious demeanour to bellow "Stay on my wheel!" whenever the partner seems in danger of detaching, or even dropping back ...
First and foremost, the weaker rider sets the pace. If practical (median speed not exceeding 25km/h), they ride in front.
Secondly, the stronger rider carries most or all of the luggage.
All the other answers about talking, advising on shifting, getting the nicer bike, signalling turns, are spot on.