How to Combine Pages on 8.5 x 11 sheet

I would probably just draw out 2 frames on the master page, select them and choose Object > Fitting > Frame Fitting Options, then choose Fit Content to Frame from the Fitting popup menu. Now you can return to the document page, and place the PDF with the Show Options checkbox turned on, so you can choose all the pages you want from the PDF. That loads up the place cursor with the pages…

Now you can just click, click, click all the pages (adding pages as necessary with a Command-Shift-P or Ctrl-Shift-P). Each time you click on a frame, it places another page and automatically scales it down to size.

There's another script that allows you to resize the PDFS as it places each page in your document. I use it all the time for just what you are trying to do.

Try here:  https://indesignsecrets.com/placing-all-the-pages-of-a-pdf-inside-indesign.php

There is a way to quickly add multiple page pdfs to an InDesign file using a scrpt that has been included in the last several versions of InDesign. First though I am assuming from your screen shot that the two pages across in landscape are the two pages shown. If that is the case I would start by rotating the entire pdf in Acrobat so that it is landscape. Since you said that the pages are already 8.5 x 11 that should work out OK.

Then create an InDesign file that is 11 inch x 8.5 inches with Facing Pages unchecked and margins of 0. In the open file go under the Windows Menu to Utilities and choose Scripts. The Scripts Panel will look like the screen shot below:

Under Samples>Applescript choose PlaceMultiplePDF.applescript and double-click. You will then get a navigation window and use it to find the pdf you want. It will ask you what page you want to begin and say page 1. If, as I said before, your pdf is the correct size all of the pages will pop into place. If they are not already the correct size the sizing can be then done in InDesign but at least the placement will have already been done.