As a rule of thumb, I would suggest structuring your data source so that all repeating groups have their own exclusive non-repeating parent. In your case it would be something like:
myFields
|- Group1Parent
|- Group1 (repeating)
|- Field1
|- Group2Parent
|-Group2 (repeating)
|-Field2
When you want to use Insert on a specific nested repeating group, like in your case, you need to specify the xpathsrc parameter. Please try this:
Insert /parent=/my:myFields/my:Group1[1] /child=my:Group2 /count=2 /xpathsrc=/my:myFields/my:Group1/my:Group2