A single provider can simplify work and create a dependency that is difficult to manage during an outage. The preparation of alternatives concerns compatibility, timing and concrete conditions. Having a second name in your contacts does not prove that the product can be replaced when needed.
I propose to distinguish standard components from dedicated supplies. For the former, a periodic availability check may suffice. For the latter, tests, adaptations or new specifications may be needed. The choice depends on the importance of the supply and the cost of maintaining a usable alternative.
We imagine a material that requires different processing at the second supplier. Replacement also involves domestic production. A limited test allows you to know these changes before an interruption, without assuming that future conditions will remain the same.
The plan should indicate when to activate the verification and who collects the information. Alternatives should be updated when changing the product. The aim is to know the company's margin of adaptation and to reduce decisions made without data at the time of the emergency.