Though this can technically apply to any stage in the game, I will be referring more specifically to the Society Stage onward. My question is if two or more dominant species on a planet coexist at the same time. That is, can they both work towards achieving spaceflight without interfering with each other’s ambitions, coalescing into the same species, etc.?

While this is possible, I believe this has already been answered/explained.

The answer depends on the time scales. As has already been discussed the more advanced you are the shorter the stages become. So I’d say that it is quite likely that once you reach society stage no species is going to evolve a lot so it would be unlikely for there to be multiple intelligent species in the society stage.

They would both have to be very tolerant, just look at humans, some (not so cool) people even have problems with tolerating other skin colour:face_with_raised_eyebrow:

I’d assume intra-species racism would go away when a species encounters another intelligence, but there’s going to be quite a barrier between the two. At least there’s the possibility of establishing communications, but that’ll be a difficulty your species may solve.

This quote regarding Warhammer 40K (where humans are at war with every other species) sums it up nicely, “Black and white teamed up on green.”