It is possible to use miles from separate accounts to book reward flights, but there must be enough in each account for each leg of a journey.

For example if you wanted to fly Gatwick - Orlando on a reward in economy:

For each person it is 42,500 miles return plus the taxes and surcharges.
That is 21,250 miles each way per person - x4 for two of you for the return.

You need to think in multiples of 21,250.

If say you had 25,000 miles in your account and your other half had 75,000 then you could fulfil one leg of the journey from yours, and three legs of the journey from the other account and therefore book the flight (subject to availability).

But if you had 20,000 miles in your account and 80,000 in the other it wouldn't work, because one account is deficient for a leg of the journey. You can't part fulfil the minimum requirement per leg from one account and top it up from the other.

Does that make sense?