Word 2016 - View side-by-side displays docs vertically

Word 2016/Win10:

Since yesterday, when I view two Word documents side-by-side, they display one-on-top-of-the-other. When this behavior first occurred, I thought it was because the 2nd doc is in landscape format; however, it's still happening w/o that factor.

Is there a setting I inadvertently changed?

Thanks,
Steve