(1) The way salah is offered outside plane (e.g. facing Qiblah and in standing position), in the same way it will be offered in the plane as well. It is not right to offer salah sitting, the salah must be repeated if performed so.
(2) According to Hanafi Fiqh, it is unlawful to combine two salah at a time except in Arafat and Muzdalifah; therefore one should try to stop the vehicle and offer salah. If the vehicle cannot be stopped, then the salah should be offered in the vehicle if it is possible. One should not combine two salah at the same time.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband