<p>I am modifying some Slic3r config parameters and comparing the results.  How can I have two instances (or equivalent: I would like to see two model windows with their associated configuration screens) of Slic3r at the same time?  I'm on OS X, but if there is a generic (e.g. within Slic3r) solution that will be preferrable.</p>

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.