Yes, you can book the different tickets from different accounts. But if you see the inventory and she doesn't you're not going to be able to book the flights from her account. If you do book from two accounts you'll be on separate PNRs but that shouldn't be too big a deal.

You will each need to have sufficient points for a whole ticket. If you need two 50K awards and you have 60K/40K in the two accounts you're not going to be able to combine them to represent 100K points and booking both seats unless you pay some rather high fees.