Prudence indicates you disable Ext. Ram during internal data access.
again, I work with the RD2/668 and do not know the MSC1210Y.
For those chips there is no such need and I can not in my wildest imagination see that such circuitry should be needed for a chip designed by reasonable individuals.

Again, when you have external RAM anyhow, why do you care whether the RAM accessed is internal or external? [this to understand you better]

Erik