In the name of Allah the Beneficent the Merciful

Is it permissible to perform one voluntary prayer with more than one intention, regardless of the  Istikharah prayer?

Praise be to Allah, prayers and peace be upon His kind Messenger, his family members, companions and followers.

Yes, it is permissible to pray a voluntary prayer and it will suffice for all kinds of voluntary prayers. However, if each prayer is performed separately, it would be better and the reward would be multiplied.