Yup they are seperate entities/states.

Is the popular thought for them to rejoin?

I would say it fits into the overal popular imagination of the "shared cultural realm" or nation I talked about but in terms of politically, as in a political union of some kind, I would say that it is not feasible or expected in the short and medium term.

The steps are being laid down though, through ASEAN and infrastructural initiatives. Economically, and culturally we are very much intertwined.

And Singapore and Malaysia are really new states, less than 75 years old, so things may change quickly.

Also, Brunei could be absorbed into Malaysia if the Sultan ever decides to in the future as that's also part of both countries' constitution.