This is a topic about which I have a fairly comprehensive knowledge but your question as it is written is a trifle unclear.
First of all, you say that the original offence was speeding, travelling at 33mph in a 30mph zone. The threshold for action to be taken against speeding drivers is (Limit + 10% + 2mph). So 35mph in a 30 limit. I have never encountered somebody seeing action taken against them below this level and as far as I know every area in England and Wales follows these guidelines. However, no matter.
The charge you refer to is laid under Section 172 of the Road Traffic Act and is for “Failing to provide driver’s details”. When an offence is detected and the driver was not stopped at the time a S172 notice is sent to the Registered Keeper (RK - who may not necessarily be the owner or indeed the regular keeper of the vehicle) asking him to provide the details of the driver at the time of the allegation. If the RK fails to reply – even if that failure was because he did not receive the request – he will face a charge under S172. The penalty if convicted is six points, a hefty fine and insurance grief for a number of years (insurers dislike intensely the endorsement code – MS90 – which accompanies the conviction). The matter cannot be dealt with by post and a court appearance is necessary. If the RK did not receive the request the onus will be on him to convince the court of this. Otherwise the notice is presumed to have been served two days after posting. So, the first bit of your question makes perfect sense and the RK (person A) would normally have to defend the matter as I have outlined.
Now the complication of Person B. Normally, if the RK was not driving he would provide the details of the person who was and that person would then receive his own request. In this case I assume (though may be wrong) that the police have obtained the details of person B somehow else and (they say) have sent him a notice. If that is so then Person B will be in exactly the same position as person A. He has an obligation to respond to the notice (even if it is to say he knows nothing of the matter) and will face the same charge if he fails to do so.
So how did the police cotton on to person B? Do A and B know each other? Is B insured to drive A’s car? Do A or B have details of the date, time and location of the original speeding offence? If so, do either of them recall being there at the relevant time?
Help me out a little and I may be able to help further. On one thing you can depend – this will not go away simply by someone saying “It wasn’t me, guv, I know nothing about it”. If a summons (or more probably a postal requisition) has been issued then matter will have to be dealt with in court.