It isn't plausible for two different relatively unrelated intelligent species to evolve intelligence at the same time, at least naturally.
The time it took for humans to evolve from rat-sized creatures to intelligent beings is a short one. 66 million years ago, mammals where tiny critters. Reptiles first arrived on land 300 million years ago. The ancestor of humans and apes was about 10 million years ago. We looked like modern humans (with the same voice box etc) for about 200,000 years.
Assuming you want civilization with cities and the like, that took place in the last 10,000 years. The one to reach civilization "first" might find and domesticate the other species. So you need two relatively random windows of 200,000 years to overlap with one ahead of the other over a domain of many many millions of years.
For this to work plausibly, you'll need coevolution. That means something about one being intelligent drove intelligence in the other.
The easiest way is genetic engineering. The "Reptiles" evolved first, and they genetically engineered a slave race. This requires "Reptiles" to be relatively high tech.
Slightly less plausible, but more so than accident, is that the "Reptiles" bred the humans to be smarter (but not to aggressive); a bit like how we bred dogs from wolves, but moreso. You could imagine the "Reptiles" finding the rough equivalent of Chimpanzees or Ouranopithecus and breeding "Humans" from them over their prehistory.
Having a climatic barrier against Reptiles being functional (for any length of time) where the Humans could be useful would both permit bred intelligent Humans to be useful (you train them to go into that territory and come back with resources), and permits "wild" human evolution (feral Humans not wiped out in that territory).
Now, metabolism wise, warm-blooded animals are going to be doing more stuff. Warm blooded creatures heat up their body in order to make some biological processes work better, and also have cooling mechanism to prevent overheating; in exchange, they burn more resources. So you might want to make your Reptiles warm-blooded, or at least less cold-blooded than many Reptiles. Reptiles might come with a cooling system but not an effective heating system, and somehow have "hot" territory where the Reptiles can function. Outside of there, Reptiles need both lots of heavy insulation and fire to keep their dwelling warm enough. In effect, what we'd call "temperate", they'd call "arctic".
This would restrict the Reptile civilization to the tropics for much of history. The domestication of the soft-body and training them to do hunting & gathering tasks would allow Reptiles to spread further away from the Tropics. They would initially be bred for being both docile and intelligent by marginal Reptile tribes.
Feral soft-bodies escape, and continue to evolve more intelligence. Reptiles breed the feral ones with their domesticated ones, breeding out the aggression and getting increased intelligence out of the wild genes.
Eventually there are 3 regions; the Hot central Reptile lands, the "Temperate" lands where Reptiles rule over Humans, and the "Cold" lands where Feral Human tribes roam.
The feral and tame humans are mostly on a one continent, isolated from the larger landmass like how the Americas is isolated from Eurasia+Africa.
Reptiles develop farming and civilization and tool use first in a "Hot" region of the super-continent, probably due to a mass climate disruption (like it apperas it happens on our planet). This also happens on the "Americas" continent, but due to the usual "Guns Germs and Steel" rules the larger continent advances faster.
They develop civilization, and during an age of exploration discover the "new world" with a mixture of soft-bodied Humans and primitive Reptiles. Where they interact, the soft-bodied Humans are slaves; they learn about how to enslave and domesticate the soft-bodies from the Reptile civilization over there. Lacking the "cold" region that the New World has, they have less fear of Feral soft-body packs. In addition, better Old World technology makes short work of the Feral soft-body geographic advantage in the cold parts of the New World.
A slave state starts its spread over the cold parts of the New World. Soft-bodies are exported to the Old World and used as servants in the Temperate parts of it. The New World Reptile civilization is conquored by Old World civilizations and somewhat merged, with its own slave-state habits.
The diseases from the high population density and trade Old World decemates the New World Reptile empire. Soft bodies exploit this. The war on them is harder than it was on the native americans of our world (as in our world, native americans where massively weakened by epidemics), but the technological edge remains huge.