There are some great tandem two seat trainers in production, like the American Champion Champ and Citabria or the Carbon Cub. Yes, they are tail draggers and yes they are fabric covered. I have successfully trained many pilots over the past 25 years using taildraggers as my primary trainers with no more difficulty than using a 152, the only difference being the pilots know what their feet are meant to do and can consistently land an aeroplane properly.
As far as fabric covering goes, it's no more costly to maintain than a spamcan, now with SIDS programs they may be more cost effective.