The crashing is very strange theres no reason why OBS should crash from a physical audio loop but you could try uploading an error log to see if someone more experienced can help you out...
If the Mac version crashes, we will need to see the crash log to see what's wrong. Here is how you get the crash log: 1) Click the Report button 2) Copy the crash log 3) Post the crash log to a site like http://pastebin.com 4) Post a link to the log in your post, along with steps to reproduce...
obsproject.com
Also can't recommend SoundFlower due to it not really being supported anymore, but if audio input is working for one device its unlikely to be OBS that is the issue here.
Also I am sort of unclear what the purpose of the multi ag audio is for, if you want two different audio devices you can do that in OBS either using Settings > Audio or by adding Audio Input layers to your Scene.