Recently I had to use MSBuild directly to automate some builds. I also had to configure some preprocessor defines based upon a configuration. This is usually done by an Argument like this

"/p:DefineConstants=MY_PREPROC_FLAG"

Nothing special here since there are enough comments on the web about that. Today I needed one Flag more and I used the commandline syntax similar to how I knew it from the IDE:

"/p:DefineConstants=MY_PREPROC_FLAG;YET_ANOTHER_FLAG"

but this one didn't work.
So the point is that if you want to support multiple defines to a project by commandline you'll have to separate them by simple spaces...

"/p:DefineConstants=MY_PREPROC_FLAG YET_ANOTHER_FLAG"

and it will be added to the (semicolon-separated) Defines from the IDE.
Good to know I think...