From a terminal window, run the command

open -n -a slic3r

Each time the command is executed, a new instance of Slic3r is created.

From a terminal window, run the command

open -n -a slic3r

Each time the command is executed, a new instance of Slic3r is created.

As per Carl's note, keep in mind that both instances of Slic3r will be sharing the same configuration files, so it will be safest not to save configuration changes while both are open.

From a terminal window, run the command

open -n -a slic3r

Each time the command is executed, a new instance of slic3r is created.

From a terminal window, run the command

open -n -a slic3r

Each time the command is executed, a new instance of Slic3r is created.