If in both case you need to have same attachment copy then you can use pxCreateAssociation activity. User can add document in 1st case and to the needed 2nd case just by using the above mentioned activity you can establish the link (even though case type is different this approach will work), no need to have duplicate document copy inside system. After establishing the link, from both case you will be able download (or) view the document.
@GunaSekaran_B Thanks for your response, The activity pxCreateAssociation links "Link-Association" rule with a work object. In my scenario, it is Link-Attachment associating to work object, I tried pxLinkAttachmentToCase, it did not help.
Following are the properties I am setting in Data-WorkAttach-File page, once I apply Obj-Save to Data-WorkAttach-File page, it will upload the stream using pzSaveToRepository OOTB activity, which in turn uses another OOTB Data Page D_pxNewFile.
.pyIsStreamBasedUpload = false
.pyAttachStream = pyContents from D_pxGetFile