The /amsg command sometimes works, depending on what network you are on. You can also specify multiple comma-seperated targets in almost any command, including /msg. So /msg #chan1,#chan2,Nick1,Nick2,#chan3 Message, is technically possible... except most servers prohibit multiple targets to reduce spam.
The simple answer, most servers don't allow it
Don't be surprised if my suggestions dont work.
- Raccoon