Well, the £6.70 fare quoted is the very cheapest AP fare for that flow, so my advice is to check out the fares for days and times you are considering ASAP before they all sell out!
The £45.55 fare in reverse is the same, except both portions are valid after 0930. (Evening peak restrictions apply out of Paddington, no evening peak restrictions out of other termini such as Waterloo/Victoria).
I would suggest the person with the Railcard splits @ Huntingdon, and the person without gets a through ticket (specifying via St Neots, so that you will both be on the same trains).
Looking at 10th Aug, 1858 from Huntingdon (1925 from Peterborough) is £6.70, same as 12th Aug. But this fare will sell out soon and the price will increase to £12.45, then £17.35...
The 0805, which is £6.70 on 10th, is up to £25.00 on 12th, but you can get the 0840 for £6.70. Other trains are £25 or £28 until 1140 when the price is £17.35...