2 ad networks in one game?

Is it common to use in a game 2 ad networks? I’ve been working with revmob till now but not happy about they’re eCPM, i set up unity ads, now I’d also like to set another ad network for the banners, should i do it? how efficient is that?

I am not sure how common, but it is possible.
I am aware of an app that is currently utilizing AdColony (adcolony.com) and UnityAds.

Additionally, you would need to review TOS per ad network to ensure this practice is allowed.

Actually you can and that is completely legal, because that’s how mediation platforms work (Mediation services aggregates the best demand offers from multiple ad networks, perform an auction between them and put the most profitable ad in your app.)
You need to create accounts and install their SDKs in your app.
If you’re unsatisfied with the the profits you get you may want to try Appodeal, as they take care about technical things and you will definitely get the maximum possible profit from all ad networks.