A popup is referring to a specific feature in the map. When multiple features are clicked to initiate a popup, there is just an overflow arrow at the top corner to allow showing the next or previous feature one at a time. Here's a sample.
If you want to cumulatively show information from a collection of features, you probably just want to display that information in a div and position it using the view's UI. You can populate the contents of the div with hitTest(), writing the coordinates in some useful format into the results div as they come in.
The problem with using View's UI is that it's not attached to the map, that is it doesn't stick to the map point but rather the screen point, so when a user drags the map or zoomins/zoomouts it's not ideal.
So is there a way to avoid this issue ?
I think in that case you'll need to listen for a click event to capture the coordinate that was clicked, then use the GraphicsLayer to display a TextSymbol of the coordinate value. Graphics in the map are fixed to a certain location and move with the map. You'll also probably want a button on the UI for clearing the GraphicsLayer.
The popup by design is a single popup at a time. Like Blake said above, the popup will let you paginate multiple features. But if you want to display more than one popup content, you can use the Feature widget and display them somewhere else on the page like this sample that displays multiple Feature widgets.