Very nice frame and features.
It looks like it has been built for time trials as the stoker handlebars position will make a rider go tucked in right behind the pilot and the rear triangle also looks short.
It has got a braze on a front mech so if you want to fit one you would have to go crossover (probably triple) or run in line but you would have to go for a 1x5/6/7 /8 speeds( depending on spacing) setup.
If you decide to go crossover you would need a tandem specific set rather than putting one together from 3 solo chainsets as the threads for the pedals on the left hand cranks are reverse.
I don't know how much you want to throw into it but Stronglight make a tandem specific crossover set. Spacycles have them at £130 which is not bad.