"write ROM in a memory position, write RAM in the position after ROM, present both words in the display... start addres=(R)OM, finish address=RA(M)"

not needed, in RAM starved designs it is often better to do the following:

If ROM first
In ROM
'ROM string",0,RAMmessageNumber

If RAM first
In ROM
0,RAMmessageNumber

Then use a lookup table in ROM to get the address of the RAM message based on the number.

'RAM message',0,0,0
or
'RAM message',0,ROMmessageAddress

Of course, the above may be followed by the 'zero byte switch' ad infinitum.

Have fun,

Erik