Just doing [100, 50] is not really helpful for breaking in the second master. Currently Glyphs respects the second number but not for breaking.
I plan to change the bracket layer syntax from [100]/]100] to [<100, >50]. So you can more explicitly say for what range the layer is made for.
So [100] would be [>100] (or [100] for compatibility with older files)
and ]100] > [<100].
And then you can specify it for each axis.