@srg36 is correct. Single source is a little risky, but it is a necessary evil while launching a new product to market. The project team does not have enough time to on board a secondary source in most projects. To mitigate the risk, the  R&D project team should order/keep 6 months plus (depending on shelf life) of stock on hand for production. Once the product is launched and transferred to Operations from R&D, the Operations group can focus on finding a second source.

Using different components from the same company does not completely solve the issue of dual sourcing, because those two components are not completely independent. Most likely the two chemicals are made in the same location. So if something happens to that production location, like you said in your post, then the production of both components will be stopped and that means that your company cannot produce the finished product. Even if the two components are produced in different locations you are still dependent on that company staying in business. Companies fail for many different reasons, and if a company that produces the key components for a product goes under then you are back to the beginning having to identify new sourcing partners. In the end dual sourcing boils down to decreasing the odds that any one event can knock your production offline, whilst souring different components can mitigate that risk, there are better ways to mitigate these risks.

 I also think it would be best to go with an entirely different supplier, both for the reasons you mentioned as well as others. Quality issues or a disaster at the manufacturing site would result in using the same supplier still having problems. In addition to this, the company or site could close down, the product lines could be discontinued, or the company could undergo a merger that harms the relationship. While some of these issues could be resolved by ensuring that the two coatings come from different sites it seems unlikely that a company would produce two products that are similar enough to serve as equivalents to each other and produce them in wholly separate sites. 

I do see the point about the different coatings from the same supplier being risky, but for the earthquake part, that would depend on the company and which branch locations manufacture which components. I think a concern leading to the desire to have more than one supplier is in case of coating A being found non-compliant, having a recall, or having manufacturing delays or issues. Not all of those issues are transferrable to coating B, even under the same company, but they are valid to include into the decision making process for choosing backup sources. In the scenario in this mini-sim, the two coatings that were chosen had the closest specs to each other and to what was desired for the device. The only coating options were from those four companies listed. In a case where two potential backup sources have very similar specs, perhaps then that's when choosing from two different companies would be preferable.