Eyyyy you caved in
I think the sentences describing their actions (like S.A and S.B share a laugh) should be italized not bolded to make it look neater. Just a suggestion.
Also
I just love fucking cheeseburgers, man.
This sounds so wrong.
ETA: As far as I understand, the painting only record the last person to enter it, right? If so, the entrance of Subject B would make Subject A disappear. For more tragic value, stop all testing, thus leaving Subject B in eternal guilt and suffering.