I'd thoroughly recommend tandeming,especially if one half of the team isn't such a strong rider.
There aren't any serious technical issues to riding a tandem,usually an hours getting used is all that's needed for general riding,though if the stoker is a solo cyclist s/he may have to compromise their individual riding style slightly and accept that the captain/helmsman is just that,as the one with the steering makes the decisions and has full control of the machine.
Stopping and starting needs a little practice but doesn't take long to master,providing the team communicate effectively.
Also an agreed cadence needs to be reached as both riders' may differ widely.
Braking will need a little practice,especially at speed and in traffic,they don't stop quite as quick as solos.
Secondhand tandems come onto the market regularly and sometimes almost new as people sometimes buy the best and find out they don't like it.Both of our high quality adult tandems(Santana and Cannondale)were bought s/hand for half their new price.
We also own a childback KHS tandem again as new and bought for less than half the cost of a new one.
The Viking ones I believe are very good value for money,and an ideal starter tandem,though be prepared to upgrade the tyres and do a bit of fettling to get it tuned up.
Thorns are a great tandem s/hand,tough as old boots and usually well looked after though heavy.
I'd beware of older machines not that some aren't very good but usually the gearing is limited and the rear OLN is usually only 135mm or less which limits wheels to MTB hubs and 36 spokes,OK if you're a light team or the stoker is light,but if you're heavy can lead to problems,also some older chainsets only allow a 28t granny ring,you'll find unless both quite strong and working well together you need a loowww bottom gear as generally tandems are slloowww climbing compared to solos.
But if the frame,wheels,and chainset are good you're halfway there.
One thing worth considering if the stoker's tall,is that older tandems are short in the stoker ETT which means they get limited viewing and feel cramped,especially if the captain is taller.
Places to look aree The Tandem club website,here and Ebay.
Have fun
