During the Heart of the Swarm campaign I've was given the advice: if you lay more creep tumors, your creep will spread quicker.
Is that the case in multiplayer game?
This is indeed true in multiplayer as well; placing multiple tumors near each other makes the creep spread faster. I have seen this mentioned and demonstrated in videos, but the only reference I can find right now is via Liquipedia:
It's true and rather simple to explain:
Don't consider creep tumors defining the area where creep might spread.
Instead, consider them as generators:
Every x seconds every creep tumor will spawn a new creep tile within its radius, if there's an unoccupied terrain tile left that is next to existing creep. Also for this it will pick closest possible location first.
If there are two (or more) creep turmors nearby, you'll just end up with 2, 3, or more new creep tiles every x seconds, which essentially speeds up the process.
Overall, creep generation (and degradation) is a "game of life" kind of simulation. If there's no tumor or main building nearby, tiles "die". If there is one, they grow.