I know there is AOC custom map for Double Arabia that starts with 2 TCs, but is it possible to design a Double Arabia or choose to build one more TC as cumans can do in feudal age but I want to build it in dark age, may be like double nomad?

There exists a Town Center limit of one (1) in Dark Age. Alternative you could create a Custom Scenario Map with fixed position and the desired numbers of Town Centers.

I want to know more about the first option, how to exceed the limit of one town center?

It's not possible, unless you create a new data mod with the desired changes, similar to our Game Data mod.

🥹 Can Do Nothing now

What you are trying to do is not possible with the original game.