Do not worry about it, the moment internal memory ends, external memory is accessed.
If, for instance, you have a chip with 4k internal ROM and strap !EA to Vcc, the processor will seamlessly run the first 4k in internal memory and roll over to external at 0x1000.

Do remember that in the above example the first 4k of the external memory is not used.

If you are trying to have some specific code in the internal and some other in the external, taht is a job for the linker, not the hardware.

I have done exactly that with the code in internal ROM and a calibration table in external.

Erik