I was wondering if Mavic 3 can record videos with the two cameras simultaneously (similarly to for example a zenmuse h20t (yeah, I know that's a 10k+ camera)) or if not, is there a chance when the sdk comes out?
No. It cannot, and to be honest, I can't imagine a scenario where you would want to. The telephoto lens delivers very different IQ than the main camera. By which I mean it's not particularly good.