If you and your partner have not ridden a tandem together, we strongly suggest trying out one or two and seeing how how you get on. The important thing is that you both enjoy it and feel comfortable and safe together.
There is a technique to safe starting and stopping on a tandem which is worth learning. When riding the important things are communication and the stoker's trust in the pilot.
Like solo bikes, tandems come in a range of styles and sizes. A tandem should suit you both in terms of sizes (usually expressed as seat tube heights like a solo bike) and lengths (the distances measured horizontally between the stoker and pilot seat tubes; and the pilot seat tube and the head tube). Older tandem designs tend to be shorter at the back..
There are several firms who hire out tandems. See our Tandem Hiring page.
Tandems vary greatly in price from a few hundred pounds up to several thousand. If you are considering buying an old tandem, perhaps because the price is low, check that spares are available. Membership of the Tandem Club can be very useful (Application Form). Through the Club you can access masses of knowledge and experience and specialist advice from Club Members. Take a look at the Club's Discussion Board