I think that at least in one place the early Sutta say that there is one Brahma (primary god) in a world system but they also speak of a world system which is thousand or manyfold but i can't recall if it is said that these do not have a Brahma god in them, they don't as i understand Buddhisms and are rather spoken of as comprised of earths & suns like our immediate environment complete with it's human kings, godly retinues & mount Merus.
I think this entire world system with it's thousandfold systems can be thought of as a single system of conceivable points of reference or observers as in special relativity.
So Brahma is max power which is a constant position and Tathagatha is occasional superpower.
At any given point there is only one ruler of the world because it's all one one conditioned system which changes as it persists.
Texts do say that there is also only one Buddha in a world system simultaneously and that a world system contracts & expands in cycles.
Unfortunately the texts do not always differentiate between a world system as to greater or lesser (they do sometimes) and therefore it is a controversy.
Some people think there can be several Buddhas in what we normally refer to as a universe, others think it cannot happen. Ie some hold that it's 1 Buddha per solar system or galaxy simultaneously.
The idea of 'sharing a world simultaneously' when analyzed in terms of electromagnetic radiation gives a clue as to how this can and how this can not work.
Ie if 'one' is thought of as 'teleporting around the universe in terms of physics' then one is traveling faster than light but one is still in what is spoken of as 'a space' where one travels at a constantly infinite/instant speed.
One such as this can not travel "in time" as one is getting old as one's perception changes as one discerns a before & after.
On other hand this teleport power opens up for being seen in multiple places because one can having emitted radiation here, disappear & teleport, faster than that radiation, to a 'there' so that one can perceive that formerly emitted radiation, as visible light there and thus even seeing or feeling oneself let alone others seeing you as radiation from here and there hits simultaneously:)
I think this is the only way one can become many.
Therefore one can see that all locations are conceived locally and are immediately accessible to a Buddha; therefore another Buddha cannot exist even a trillion lightyears away nor in any other conceivable location for as long as the life-force of a Tathagatha is a factor in the world. That is because, as i explained, all locality is immediately accessible to a Buddha as it is conceived and that obv faster than light as quantum entanglement demonstrates.