If you don't mind paying the £25.00 cancellation fee on a booking you've made but don't end up using, I'd say it is a good idea to book the one seat you can see know, and hope a second seat opens up too.

I'd say you have a greater chance of bagging one seat now, and a second opening up nearer to departure time, than letting it go and hoping two seats open up later on.

There will be those who object to you intentionally blocking award availability by doing this, however if your conscience can overcome this ( - which a lot of people's does!) I'd go for it.