The current-page-number tag is Cmd-3; when you want to tag the number of the page containing the previous text box in the linked boxes, use Cmd-2; Cmd-4 for the next.
So if you want to have both previous and current in one single box, you must tag Cmd-2 | Cmd-3 PLUS you must link an empty box set on the the left-hand page, insert a jump to next box and you're done. Am I clear enough?
Mind you cannot link text boxes from a left-hand page to a right-hand page in Master page. So all you can do is preparing all that on the MP and then manually link the boxes in the document.
Another way is to draw a text box on the left-hand page and drag its right edge accross the spread. Then right indent the Cmd-3 tag and it will show the left page number on the right page. Magic :-) Then draw another text box horizontally aligned to the 1st one but entirely positioned on the right page and type | Cmd-3 in. Tweak the position of the whole stuff and you are done. Hope this helps!