"Ik sta ertussen."
Translation:I am standing in between.
Because, depending of the usage, 'betwixt' is either archaïc or mostly unused. This is the first time i hear of this word in 10 years of english. I don't feel like hearing it in a natural conversation. I guess Duolinguo try to teach us how to speak naturally instead of allowing every shakespearian possibilities :) BTW, have a lingot for the new funny new old english word
Intussen=in the mean time. "Tussen ... in" is accompanied by what/whoever the things/people on either side are. Ertussen or ertussenin usually refers to something mentioned before.
Intussen was de politie gearriveerd=In the meantime, the police had arrived
Jij mag tussen opa en oma in zitten=You may sit between grandpa and grandma
Opa en oma zitten op de bank, jij mag ertussen=Opa en oma zitten op de bank, jij mag ertussenin=Grandpa and grandma are sitting on the couch, you may sit in between them.