@Salil the only landing page capability where you can "select multiple" is when you have a List of Cases (List View) and that has Bulk Actions enabled. 

However, this is about selecting multiple cases and then taking individual actions on each case, in bulk. For example, Approve action, would just approve all the items in the list - in the background it is processing each one by one with that action. In your case you are trying to select each item to take a single action "compare".

So, in this case, you effectively have a Data Reference (Multiple Records) that you want to Compare against and you'd need a case to do it. In the case of a Landing Page > List View, this does not support multiple selections of data records to launch a single case. You would need to build a DX Component to do that.

• In my head, the DX Component would handle selecting the items to compare, creating a "Compare Tool" case, and passing the selected items.
• From there, Pega OOTB case management would take over.
  • In a stateless architecture, you'd need the case to do all the processing (data transforms, data modelling etc). You'd likely also need a case to do other things in the future - maybe we want to spin off other cases/processes when we notice something during comparing.
  • Trying to do this fully inside a DX component would be costly and hard to maintain

Compare DX Component (for Case)

You might want to have a look at this example from the Pega GitHub (open source) 

• https://pegasystems.github.io/constellation-ui-gallery/?path=/docs/temp…
• How to implement the compare and select pattern in Constellation UI | Support Center