Dawes double edge is a good starting point
Front suspension is pretty essential for anything offroad – as is a sus seatpost for the stoker. Also the advice is always try before you buy – some couples just cannot get on with one. Welcome to have a shot on ours if you are anywhere local or lost of place hire them / offer demos. JD tandems is a well rated shop who do tryout days.
the fork really needs to be very tough – they get hammered. Mine is a old cannondale with a ‘zocchi Z1 lite fork modified with a reinforced steel steerer and internal mods
the tandem club has a useful website with details of places to try tandems.
Edinburgh BTW if you want a shot on ours.