Just a suggestion, assuming the dimension's model value is in inches, and you want your 'alternate' units to be milimeters. You could try something like code. It obviously isn't customized to your situation, but the methods are there. This should modify all dimensions on all sheets of the active drawing to showing the milimeter value of the ModelValue right below the ModelValue (again assuming the ModelValue is in Inches).
Dim oDoc As DrawingDocument
oDoc = ThisDrawing.Document
Dim oVal As Double
For Each oSheet As Sheet In oDoc.Sheets
For Each oDwgDim As DrawingDimension In oSheet.DrawingDimensions
If oDwgDim.ModelValueOverridden = True Then
oVal = oDwgDim.OverrideModelValue
ElseIf oDwgDim.ModelValueOverridden = False Then
oVal = oDwgDim.ModelValue
End If
'You could use a MessageBox or InputListBox here to ask which Alt Units you want to show.
'You could also set up the math factors as Local Variables for the other Unit conversins here if needed.
'This is just converting adding a new line under the existing ModelValue, then converting its Inches to Milimeters.
oDwgDim.Text.FormattedText = "" & vbNewLine & (oVal*25.4).ToString & "mm"
Next
Next
I hope this helps.
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Wesley Crihfield

(Not an Autodesk Employee)