The title is a bit different to the question, but there's a reason to that. I am currently in the worldbuilding stage of creating an alien planet, mainly the wildlife, and I may have run into a problem. Back to the title, at one point on Earth, Dinosaurs and large reptiles filled the majority of niches (I'm talking specifically land-borne life and excluding invertebrates). After the mass extinction, the empty niches were filled by mammals and birds. focusing on mammals, they were only able to fill those niches once the dinosaurs had died off and left them behind (correct me if I'm wrong).
On my fictional planet, lets call it Xenoterra for now, there are two dominant groups of animals, the mammal-like, quadrupedal group, and the more reptilian, six-limbed group (the niches usually filled by arthropods and other invertebrates on Earth are filled, on this planet, by very similar animals). If we were to think of these two groups as the equivalent of dinosaurs and mammals, (birds falling into dinosaurs because that's what they are) could they have evolved at the same time, filling the niches but not competing to the point of one being dominated? Note: I'm not including the difference in size into the equation as the dinosaur and mammal part is just a comparison, the creatures in question have the same range of size and adaptability. The question uses an Earth example but in no way goes by 'Earth rules' but the ecosystem is similar. My creatures are not dinosaurs and mammals, I'm comparing them to dinosaurs and mammals because of the similar circumstance.
this question is definitely open to editing so if you want more info tell me. I might be slightly late to answer though.