The BA 2-4-1 companion voucher is quite possibly, the most valuable perk offered by any UK credit card (more information about the BA credit card options can be found here).
Currently, there are two flavours for the companion voucher, the one issued with the free BA credit card that has a validity of only 12 months, and the one issued with the BA premium card that has a validity of 24 months. In all honesty, if you are spending enough to trigger the companion ticket on the free card, you and your travel companion would probably be better off either getting two premium cards between you, or 1 premium card and a different rewards or cashback card.
Using the companion ticket, means that you only need to pay the Avios points for one return fight, you will still however, need to pay the taxes and fees for both passengers (currently around £500 per person for a premium cabin). This means that a long haul business class flight will only cost you ~100,000 Avios points (an amount that is fairly easy to achieve in a single year, find out more here) rather than 200,000 Avios.
The value of the 2-4-1 voucher, comes from long haul premium cabin redemptions. To get these however, you will likely need to book a whole year in advance; hence the companion ticket offered by the premium card, being better suited to these types of redemptions.
The quickest way of checking for availability on any given route, is through the reward flight finder (here). Simply enter your starting airport, destination airport and flight class into the relevant fields and the redemption finder will show you all the availability over the next year.
If you know when you want to go and have specific dates in mind, then you are best booking as soon as the flights are released (355 days in advance). In this scenario you should book the outbound flight first (at the time of its release 355 days before departure), you can then amend your booking to add the return flight when those dates are released.