At common law, liability cannot be apportioned among tortfeasors, which is why most jurisdictions have enacted legislation like Ontario's Negligence Act to permit contribution among a group of wrongdoers. What is less clear is whether liability may be allocated in circumstances other than ones involving negligence, given that some of the statutes refer to 'fault' rather than 'tort' (as in BC, Alberta and Ontario). Does 'fault' encompass liability for breach of contract? Canadian courts have been all over the map on this question, judging by the Alberta Court of Appeal's canvass of the jurisprudence in Petersen Pontiac Buick GMC (Alta) Ltd v Campbell, 2013 ABCA 251.

The issue in that case was whether contract damages could be divided equally between Campbell, a lawyer, and his client, Petersen Pontiac -- each party having breached a separate contract with the plaintiff that caused the same loss. (The lawyer was also liable in negligence.) Madam Justice Hunt reviews the inconsistent authorities and the academic writing on apportionment, concluding that while 'fault' in Alberta's Contributory Negligence Act might not be broad enough to cover liability in contract, there ought to be 'no impediment in contract law to dividing liability between two parties who have breached two different contracts, causing the same damage to a plaintiff.' This finding is based on considerations of policy and is also supported, in the judge's view, by equitable principles.

http://www.canlii.org/en/ab/abca/doc/2013/2013abca251/2013abca251.html