What you are effectively generating is a pair of HTML TABLE elements. And, just as in HTML, the JSF equivalent auto-sizes columns.
I presume that you are using PrimeFaces, and I'm not that familiar with it, but the stock JSF h:dataTable tag has a columnClasses attribute that can be used to list a set of classes that regulate each column. If these classes include a width specification, you can get consistent widths by simply using the same list of columnClasses for both panelGrids.
Loudly announcing something is true and finding out you're wrong makes you feel foolish.
Finding out you're wrong and refusing to admit it makes you LOOK foolish.