I agree with what Roger said. I can see no reason that tandems would be any less sensitive backing up, and I have reversed with many a different configuration of trailer. The longer the easier if you have the room, and even easier if they are a 5th wheel.
19' I am getting is tandem. They could have gone with a single axle (dry weight
2,600 lb). Some of the negatives to tandems for me are increased weight
, increased maintenance costs (though a set of tandem tires will usually last longer then those on a single axle), and potential 'scuffing' turning real sharp while reversing (though this can usually be avoided.) Positives include more stability on rough stuff and in wind, safety in the case of a blowout, potential carries more weight
(though this really is not required for a lightweight trailer.) It is one of those things where there are good points either way, you just have to decide which you prefer.