Imposing US Letter pages to a US Ledger sheet
Q: Is it possible to put two US Letter pages (portrait) to US Ledger sheet (landscape) in the step and repeat mode?
A: Sure, see a step-by-step guide below.
Let's create a sample four pages document of US Letter size:

Now create a new project in Imposition Wizard and load the document there. You will see something like this:

It is OK, because we are yet to adjust some settings to get what we need. Open the "Sheet" page and change sheet's size to "US Ledger". You should see the following:

Now switch sheet orientation to "Landscape" and you will get what you wanted:

We don't need to setup step and repeat, because this layout is used by default. Here we can see the second sheet with two second pages of our document.

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