They could eventually through interbreeding.
What if a particular species were separated by great distances?
That would be a barrier to interbreeding.
If they were in close proximity to one another, would genetic drift affect all of them at the same time?
Genetic drift is not an "effect", it's a result. Nor is it something that can be coherently described as happening "at the same time". It's what happens when differential reproductive sampling occurs across generations.
It can be reduced to simple mathematical models based on population size. If I remember correctly.