No, either user can tap on a qube and spend money with their own debit card. You do not need your companion to also open their qube at the same time for you to be able to spend money. In fact, you cannot both spend money from the same qube at the same time, but you can spend money from different qubes at the same time.
For example, one of you could be checking out at the grocery store, while the other is getting gas, and both "Grocery" and "Gas" qubes will work. This works because an opened qube is linked to the app user's debit card.
When a user opens a qube, only their card can be used to spend from that qube. If the primary user opens a qube called "Gas," and the secondary user swipes their own card at the pump, the transaction would be declined. Only the app user's card can be used to spend from an opened qube in their app.