The simple answer is, you cannot book an award with two accounts. An award has to be booked with one account only.
You can book awards for anyone with any account, so the above restriction doesn't stop you from booking tickets for each other.
Hopefully you have the right amounts in each account, so you can book some of the tickets with one, and some with another. Keep in mind booking one-way tickets is an option to split up the miles used further.