It's actually pretty simple in Studio One. Put Presonus Compressor on your stereo guitar track. Then on the SEND of the Vocal Track, when you go to add it, you should see the option for "Sidechain". Select the Presonus Compressor, and then BOOM.
Adjust the Threshold to get a feel for how hard the compressor will hit (or duck) when the vocals are present. Play with the ratio and release to really hone it it.
Be very gentle especially if you are using your vocal to duck the guitars, because both are very dynamic audio source and might get a little pumpy, if you aren't careful (unless that's what you're going for!).
