With thumb shifters on the drop bars on a Duet, 27" tires and 18 speeds, it indicates one of the very early Burley tandems.
If it is in good condition, $700 or less would be a decent price. They very early ones sold for about $1300, brand new and were an excellent value.
If it fits, I'd go for it.
Pedal on TWOgether!
Rudy and Kay/zonatandem