Earlier quoted context omitted.
Tiny nitpick: complement means a good in an adjacent part of the supply chain (e.g. batteries and EVs are complements), you might have meant 'competitors'.
From the point of view of a customer, a complementary service would be a product that fits the same niche and is reasonably substitutable.
According to the Cambridge dictionary, there are a couple of variants depending on your dialect:
- "to make something else seem better or more attractive when combining with it" - e.g. "Strawberries and cream complement each other perfectly."
- "to help make something or someone more complete or effective" - e.g. "She used photographs to complement the text of the news story. "
The only way I can make any sense of your words is to assume you're trying to argue that two competing but interchangeable services "complement" each other when used simultaneously because that might allow a customer to mitigate risk in a similar fashion to a multi-cloud setup. That's a pretty big jump to make from what you've actually said though and would really need further explanation for anyone to understand.