Do you mean off or on road? I can't think of anything I'd like to do less than off roading with Mrs Boblo…
SJS have s/h tandems in. Might be worth trying one of these first? If you get in to it, you can then trade up.
I went in via this route. Bought a s/h Dawes Horizon Twin from SJS for £500 then a couple of years later went for the made to measure George Longstaff. £££'s though.
We're really into it now. Just got back from French Channel to Med (Caen to Montpellier). Long distance touring is our 'thing'.