This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
Sometimes. Problems have been reported so make sure you get a
money-back guarantee before ordering. MicroMac's SIMMdoubler II will
double the SIMM capacity of all Mac II models and the Performa 600;
i.e. it lets you fit two SIMMs in each slot on your logic board. It
also lets you use standard 4x8 SIMMs in the Mac II and IIx rather
than the more expensive PAL type normally required. At $140 street a
set of four is a little expensive but perhaps worth it if you have a
let of extra one meg SIMMs. Sermak Technology's (800-209-7126)
SimmStacks will fit two 30 pin SIMMs in one 72 pin slot. Sermak also
sells SIMM doublers that fit two 72 pin SIMMs in one 72-pin slot.
Since the extra height of the SIMMdoubler provides more leverage to
exert force on the SIMM sockets, some users have broken SIMM slots
when installing these products so be careful. Furthermore if you use
more SIMMs than you have sockets, you'll be drawing more power into
them than your Mac was designed to supply which might lead to
problems down the road.