The first thing is the important one. 2 cities that are earlier at size 10 will allow you to get an advantage, like you i. e. get enough units and can attack someone earlier. From this guy, you again conquer cities, which you wouldn't have in the other case, so instead of units, you now got whole cities. You repeat this a few times, and you have won the game, earlier than the other strategy ever could have. This concept is known as snowballing , so the single snowball that starts and ends as a lawine.
It's very hard to give you advice, because you can't use Slavery with your mod. With Slavery not being possible, it's attractive to grow the cities, so normal scenarios in where you i. e. share certain resources, to work them from a different city after having whipped another city that us too small now, don't work in your games. The whip is actually the main reason to keep cities smaller, because smaller cities need leaa food to regrow, therefore, whipping smaller cities oa more efficient than whipping larger ones.
What you should also not forget:
2 cities have twice the Traderoutes
The maintenance will only be higher until you've reached the limit of cities, at which the maintenance is capped
When being far advanced, like when having Corps, every city produces a benefit from TRs only, so 2 cities will generate more Commerce than 1
With Sushi, you can have almost twice the food in 2 cities, because Sushi can be spread to both
Same for Mining with Hammers
Maybe the best advice to give you is, that those cities are best, that bring you closer to your goal. The most imprtant thing in decisions, is the opportunit cost. When you want to settle, but have strong units, the units are better because they are cheaper and get you larger, well developed cotiesr instead of size 1 cities, so try to know more about your options, and evaluate them against each other.
Hth.