Hi Jill,
So, I'm really just guessing and I'm not an authoritative expert on it, but my guess is because each named user creates their own Web App ID that is used for installing and, since the web app ID is embedded into the WAB software (or is it the app itself?), that is what is blocking the 2nd person from seeing the first.....even within a common http://xyz:3344/webappbuilder The WAB still has a tie to AGOL, with the web app ID, even if it's not a direct online web app and even if you are using local layers/services for the basemap.
Legally each user has to be a named user (i.e. we're not supposed to share). But I'm wondering if the web app ID can be manipulated in such a way in the WAB or app that it can be shared? Again, I'm just thinking out loud here and I'm not sure if this is legal use of the WAB software and API since it is still a esri product, even if provided free for us to use. Really, the AGOL or Portal is the licensing. (in my opinion).
This is an interesting topic of discussion, thinking out of the sharing development of WAB. There are many that are much more knowledgeable than me when it comes to customizing, with Robert Scheitlin, GISP being my first go to guy (and the Flex guy too)....although there are many others. I'm sure others will/can chime in. (or some one from esri)