Apple Performa 6400/180 - Manual (Page 41)

Basics Dual In-Line Memory Modules (DIMMs) - 39
Performa/Power Macintosh 6400
DRAM DIMMs can be installed individually. However, if you
wish to take advantage of the computer’s interleaving
capability, which provides maximum performance, you
must install the DIMMs in matching pairs.
Memory interleaving allows the computer to read or write
to its memory while other memory reads or writes are
occurring, thus providing for faster performance.
Note:
DIMMs purchased from different manufacturers can
be paired as long as they are the same size and speed.