The above macro by @DanThomas, and the below script by @ccstone are great tools to toggle among multiple macros for editing.
But I agree with the OP, being able to open multiple macros for editing (or at least copying actions) at the same time would be great.
Another great tool by @DanThomas, which I use many times a day, is:
MACRO: [KMFAM] Favorite Actions and Macros