One creative and very workable way to do all this is to plan as such:
Find a friend, family member or co worker whom you trust or can develop a trusting transactional relationship with and see if that person has enough miles for the 90k ticket. Have them book for one person, and do your other booking with the acct that has the 100k miles in it.
Now you have both tickets. your helper person has given you an award worth the 90k on the airline you are using.
You owe them the same.
Doing promos and other sign ups in here (such as credit cards, coins, checking accounts, etc) you can bring that 80k acct up to 90k in no time.
once you have that much in there, your friend can have you give them an award (or awards) that total to what you owe them.
The idea is that you may need to book NOW, and that friend has the miles NOW, but you do not. So if you get them to book for you now, you get what you need without losing the flights you want, and you still have time to work on giving back what you owe to them later.
Anyone can book an award for anyone else. This is perfectly legal. Airlines do not mind it--and they all know that in return, you might be booking one for that friend again later one--but what they do not like is the selling of awards or other solicitation--especially online. There is no reason, therefore, that they should even inquire as to why John Smith booked your wife's award ticket, and vs. Why, you could even do it in such a way where you have someone book you something on say, United, but they want you to 'pay' them back with Delta.
This, in my opinion, remains the easiest and cheapest way to get the ticket you want when you need it. It's all about networking too.
While mile transfers between accounts may cost a lot of money, the ability to book awards for friends only has one cost: Your proper planning and administration of this process, and of course trust on both ends. Screw all that up and it all goes up in smoke real fast!
No matter what you do, or how you get your two tickets, it is imperative that you later contact the airline and give them the record locator numbers of both ticketed awards. Have them 'join' these, or give you an entirely new one that becomes the new number for both ticketed itineraries (This is especially important when using awards from various sources for the booking of childrens' tickets, by the way, because when you finally check in to fly, children must be accompanied by an adult. If you fail to simply call in and take care of this rather easy step with customer service before your flight, you will have problems checking in online, which can be a pain if you are traveling say, with small children overseas)
I hope some of this was useful.
MM