Normally on an 8051 the eprom exists in CODE space (enabled via /PSEN) and is read only.Ram exists in XDATA space enabled by /RD or /WR. Therefore you can have eprom exist at the same address as the ram but they exist in difference memory spaces and thus don't clash. I dare say you want to do some monitor or boot up memory swap so you can download code to ram - you can do this but you will need some hardware logic for this. I suggest you do some reading on how the 8051 works with memory and this should give you enough information. I did a similar thing on an older 8051 board I designed with external flash memory. I used a GAL logic device along with two port pins to perform the swapping.

Nowadays, I would just use a device with internal flash - much,much easier!