(b) That's wrong. In personal injury cases. when two or more persons join in wrongful conduct which causes harm to a plaintiff, they both become liable as joint tortfeasors.
For example, in Henry v Thompson [1989] 2 Qd R 412, three policemen were held to have jointly contributed to assaulting P. One defendant jumped on him, one urinated on him and one stood by blocking his escape. All three policemen were joint tortfeasors.
Joint tortfeasors can be sued either jointly or individually for the harm they cause. Either one may be liable for the total amount of the loss. The plaintiff can recover the full amount from either one of them, or can recover an appropriate amount from each tortfeasor according to their blame and what is 'just and equitable'.
How to apportion liability is governed by legislation, e.g. Part 4, Civil Liability Act 2002 (NSW).
In Brooke v Bool [1928] 2 KB 578 (on which this question is based), it was held that when A and B began searching for the leak by lighting matches, they were acting under a common design. Note that in this case, the damage was to property. But under current legislation in Australia, joint is only available for personal injury claims,