What you could do is use miles from each account to get the reward seat/upgrade, BUT it has to be in equal amounts. e.g if you need 100k miles for the reward then 50k from each account can be spent for it. (have done it 3 times for flights before)
You can't split the amounts say 70k from one and 30k from another account, so if you don't have the equal amounts then you will need to buy the extra miles to make it so.
You will need to ring FC to do it and you both need to be present to authorise the deduction from each account (unless you are authorised users on each others accounts).
Word of warning though, the last time i tried to do it the FC agent said it couldn't be done that way. After speaking with the supervisor she apolgised for the incorrect info from the agent. So if you get told no, try someone else.