I’m trying to convert two notes to a single 8th grace note (hope this is clear).
Both selected notes start at the same position (27. 2. 2. 107).
The workaround is to:
1 . Delete one of the grace notes and position the remaining one at " x. 1. 1. 0 ", then
2 . Click the note and drag while holding down option/alt + command/ctrl keys to add the other grace note (on the same stem).
I need a workaround to achieve what you don’t want to get! 
I dare you try to do the same thing in my original project (fragment of a project…).
Steel Guitar - problematic score passage.cpr.zip (2.2 MB)
If you look and zoom enough in Key Editor or List Editor you will notice that your two grace notes are not exactly positioned at the same tick (~107 vs ~108), because these are presumably recorded notes and their positions are not accurately displayed in Info Line.
Change e.g. the lower grace note from x.x.x.107 to x.x.x.108 and you’ll be done.
