DUAL DIMENSION 1 DIM ONLY

ANSI standards dictate that if you have dual dimensioning on any single print in the package then it's required on all sheets - unfortunately. PTC follows ANSI standards.

See this thread where that text was taken from for more comments.

https://community.ptc.com/t5/Detailing-MBD-MBE/mm-dimensions-in-drawings/m-p/235075

There's no way to turn on dual dims for a specific dimension, but if you desperately need to have the alternate units on a dimension, you could calculate the values using a parameter, then add the calculated values to the dimension by editing the dimension text. For example, say I've got a dimension D1 in inches, and I also want it in millimeters. I can calculate the mm value as something like:

D1mm = D1 * 25.4

Then, in the text of my dimension, I can cause the calculated mm value to show by making it look like:

@D
[&D1mm]