T21 and T42. Tandem two seaters - T31, Blanik, Bocian, K7, K13, ASK21 series german gliders etc, have panels in both cockpits. How else would you be able to teach people to fly?
same way that you do in a Piper Cub. Peer round the students head when you need to, otherwise fly the attitude. Most two seaters do have some sort of panel for the instructor, however. Not many countries allow gliders to cloud fly. France doesn't UK does as does Poland. And yes, I am a glider pilot, ex instructor on glider and power.
If cloud flying isn't legal then not much point fitting cloud flying instruments.