There is no way to assign multiple values to the MACHINE variable. You could run BitBake several times with different values for MACHINE though.
One way to do this would be not to set MACHINE in local.conf but to set it externally and pass it to BitBake as an environment variable.
MACHINE="machine1"
export MACHINE
BB_ENV_EXTRAWHITE="MACHINE"
export BB_ENV_EXTRAWHITE
./bitbake helloworld-image
This way, you would get images for each machine in separate directories under tmp/deploy/images .
You could then automate building for several machines with a shell script like this:
MACHINES="machine1 machine2 machine3"
for MACHINE in "$MACHINES" ; do
...
done
You can find more information about BB_ENV_EXTRAWHITE here: http://www.openembedded.org/wiki/Advanced_configuration#Whitelist_Environment_Variables