I have often done this with my SO or my children, booking myself with a revenue ticket (for the EQM and RDM) and them with a miles ticket. As a 1K, I can guest someone into E+.

I don't think it makes any difference as to the order you book - as long as you are careful that booking a seat for yourself doesn't eliminate the mileage inventory, or vice-versa. When it's been a little critical, I've always booked the mileage one first, so that, should that eliminate my revenue fare, I can always change it.

Seating is the real problem. After booking, call and get the mileage ticket's seat next to yours and request that a note be put in both PNRs that you are travelling together. That should work and they shouldn't switch your companion from E+, but keep an eye on it nonetheless.

The real problem comes with any route operated by a 777. There will inevitably be at least one, and perhaps several, equipment swaps in the week leading up to the flight. These are handled very poorly - there's an extensive thread on this - and the bottom line is that you will get separated unless you are very lucky.