How to include T&Cs on the back of every printed page in a PO form

You did not specify whether you are using SAPscript or Smart Forms. Technically, you can do it with either tool, as both offer page flow control and duplex printing options. Regardless of tool, at a high level, in your pages definition, you would set the duplex switch on for page one. Then create a separate page with the T&Cs as page two and then link it to page three (the overflow page) -- which in turn links back to page two. However, without getting too deep into these details, I would ask a more basic question:

Why?

In other words: Do you really need to do this?

I realize that this is essentially a business question, so it is something to discuss with your functional counterpart. Printing terms and conditions on the back of every page of every purchase order is expensive in terms of toner, speed and network traffic. And of course it requires buying printers with duplex capabilities. It is also restrictive; for instance, what happens when you need to fax or email a PO to a vendor?

There are a number of alternatives to consider. If the business will allow it, my recommendation is to create a single-sided form and work with your legal department to craft a simple tagline at the bottom of it which incorporates T&Cs by reference (e.g. 'see our website...'); in this way, you can eliminate the T&Cs pages altogether. I have worked with several clients that have taken this approach, some of which originally planned to do what you have been asked to do.

Otherwise, as a fall-back option, consider a dedicated printer tray with T&Cs preprinted on the back of the pages loaded in the tray. In this case, of course, any POs faxed directly from SAP still will need an end-page stored in SAP with the T&Cs on it, so that they accompany the PO.