To use miles from two accounts simply call VS FC and have both of you named as designated users of the others' account. You have to be related / same address to do this.

However, note that while you can book a redemption award by dipping into both accounts, each must have sufficient miles for one part of the trip. For example, if you use miles to upgrade from PE to Upper Class, it's 40k total and 20k each way, so each account needs a minimum of 20k, you could not do 15k from one and 25k from another.