If you are building the graph from the start and you know which columns you wish to have as separated, you can hold shift when you drop them to the axis and it should separate them. It should look something like this

and this is the normal without holding shift

Otherwise dropping them to correct location (like @Mark_Zwald already did mention should do it:
Do not drop them here (will replace)

or here (will add to same axis)

but rather to top or bottom of axis


Essential Graphing from JMP Help is a good read, here is direct link to this topic Essential Graphing > Graph Builder > Work with Axes > Create Separate Axes in the X and Y Zones
-Jarmo