Yeah, I do this; 2020 M1 13" MBP; 4K external monitor and iPad Pro on Sidecar, plus I keep the laptop open so I have three screens. I find Sidecar a bit flaky (it disconnects or freezes a bit too often to be breezily accepted), but with a big iPad (mine is basically A4 paper sized) and SwitchResX in play, it's not bad at all.
Edit: ... and having said all that, I'm looking forward to upgrading some time in the next year and going back to two "proper" external monitors. I'll still love using the Sidecar'ed iPad Pro as a second screen when working in cafes, though...
1: https://www.youtube.com/watch?v=P_pBL2fMux0