Under Windows you can simply run two instances of purebasic/spiderbasic - but on mac it's not so simple. Anyway, you offen want to have two code windows side by side.
In Sublime Text there is a great feature - when you have multiple tabs open you can simply drag one of them somewhere to you desktop and a new instance of the application opens with the sourcecode opened. That would be a great feature for the purebasic/spiderbasic ide as well solving the mac problem with two applications as well.
BTW: if you want to run two instance of the IDE at once you can go to the console and start a second one like this:
open -n /Applications/SpiderBasic/SpiderBasic.app
open -n /Applications/PureBasic/PureBasic.app
but with the second instance you get a warning that "A previous IDE session seems to have ended improperly" - I have not tested if this leads to any problems when using two instance of PB on Mac at the same time.