We find that sometimes we can get good M+M fares, but at other times they are hardly any cheaper than the regular discounted fares. For January we have used 30,000 miles for a Companion PE flight, so they have cost £550 each. The M+M fare was £690 each.
Although they say that you need to pay the full flexible fare to get a Companion flight, that isn't always the case. This time, for example, we've paid the discounted fare, i.e. the amount they quote if you check rates online and input "Premium Economy (lowest)".
You can't check rates or book online; you have to phone the Flying Club.
If you have insufficient miles to book a Companion flight, but enough between two of you, you can each book one leg. One of you will be in the revenue flight and the other in the reward flight both ways. The first time we did the split miles thing, I'd assumed that I would be in the revenue seat one way and dh the other, but to do that, you'd have to book two single flights which would cost much more than a return flight. This is important to know, because the person booked in the revenue flight will earn miles and tier points, whereas the person in the reward flight won't earn either. I will lose my Silver Flying Club status this year as a result and it's much harder to earn it back than it is to keep it.