You almost always can use multiple resolver operators. Whether it's a good idea depends on your requirements and priorities.
You get a combination of the features offered by both providers. Notably, 9.9.9.9 blocks malicious domains, and 8.8.8.8 does not. If you use them both, sometimes malicious domains will resolve and sometimes they won't!
8.8.8.8 also supports EDNS Client Subnet, which can improve CDN performance, while reducing DNS privacy and performance. 9.9.9.9 does not, though Quad9's alternative 9.9.9.11 resolver does.
Some DNS services provide detailed logging. Conversely, some promise privacy. Most people probably won't want to combine the two.
DNSSEC can be an important difference. If one resolver validates and one doesn't, you'll be able to resolve bogus domains (sometimes slowly). (The resolvers mentioned here all validate, and I don't know whether that is important to you anyway.)
If you care about certain features, you'll want to consider the trade-offs and choose which resolver(s) to use carefully. If you don't, you can go wild.