Problems W Dell Insperion 5100 Laptop ram not working

My brother is trieng to put 2x512 pc2100 in his laptop and the com will not startup.  1x512 ram stick will also not startup.  Now if he uses 1x256 2100 and 1x512 2100 he gets startup and 768 ram.   Ive read threads and specs and they all have problems and suggest using certain company rams and i find this to be a stupid solution because if the 512 works with the 256 then it should work all together.  Also i dont have a bios screen available to look at because hes livin out of state and I have no idea what changes to make in the bios to make this work.. help a brotha out    :)