SAS MA allows you set up different subject levels and data sources in a data mart, thus enabling the business users to run both types of campaigns (subscribers and dealers) without having to switch between the business contexts.
As long as business users and the general process are mostly the same, the best practice is to use one business context for all.
Different business context is more of a multitenancy feature when completely different lines of business, business groups etc share the use of the solution. For example, when SAS MA in a bank is used both marketing (sales) campaigns and debt collection campaigns. Debt collection campaigns may be managed by a completely separate business team, with a different process, different channels, etc - then separate business contexts are most appropriate.
Without knowing more details (what is your industry, what is a "campaign for dealers", etc), it is hard to decide which approach is more appropriate in your case.
At the same time, what you are trying to achieve seems a completely different type of campaigns with significant differences - different data sources, different subject audience, probably different channels, etc. There might not be a single "how-to guide" or a similar material that covers all aspects, challenges and pitfalls of such implementation. I'd recommend you find and engage an experienced SAS CI solution architect either from a third-party experienced consultancy practice (or SAS Professional Services) to help you design the most appropriate best-practice solution according to your business requirements.
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Dmitriy Alergant, Tier One Analytics