For a long time Visio has supported stencils and templates in two different measurement system; Imperial (US) and Metric. Each template had a US version and a Metric version. So why two templates? The US templates were in in, ft, yds and the other imperial units and the Metric templates were in mm, cm, m and the other metric units. There was also an isue of the physical page size; Some of the standard sizes for the US are letter, legal and tabloild while the metric sizes were the A format sizes like A3 and A4.
So is it possible to have a single template that will handle both Imperial and Metric units and paper sizes? Yes, with a few minor changes, you can combine both units.
One method to do this is to add a user defined cell to the document and use that variable to determine the system to use. To add a document level user defined cell, right click the top entry in the Drawing Explorer and select Show ShapeSheet or right click the drawing surface outside the drawing and hold the shift key while selecting Show ShapeSheet. If you do not hold down the shift key, you will get the shapesheet for the page. Add a rowl to the User Defined Cells section and name it something like UseMetric. Use a value of 0 for Metric units and non zero for Imperial units.
On each page set the PageHeight, PageWidth, PageScale and DrawingScale with a formula like.
For Page Width set the content to: = IF(TheDoc!User.UseMetric=0,210mm,8.5in)
As you change the value of UseMetric, the PageWidth (and other qualities) will reflect the selected units.
This will give you a document that will switch between a letter size drawing in inches and an A4 drawing in mm.
The next step would be to add unit agnostic formulas to your shapes.
If you already have background pages created in Imperial units and metric units, rather than trying to merge the two with formulas based on TheDoc!User.UseMetric, combine the two background pages by applying each page to a layer and use TheDoc!User.UseMetric to control the visibilty of each layer.