wrote in message
I'm sure I've come across posts from people here who seem to fly more
than one type of plane at a time,
It's not allowed in my country, and was interested in knowing whether
India alone is archaic or do similar rules exist elsewhere too?

Those rules descended from the United Kingdom. They still have similar
rules. If you are IFRcurrent in a B-737, you still have to take a checkride
to be IFR current in a C-421. Last I heard, the CAA allows only 2
type-ratings at any one time.

D.