I've done this. I used a dummy ability as second ability.
I added the building options of both building 'abulites' to the building list in the object editorof the appropriate worker unit.
At game start I disable the buildings supposed to be in the second menu.
When the 'dummy' build is clicked, I disable the main menus buildings from being build, then enable the second menu. Then I force the unit to open the build menu 'force UI key' I think.
And vica versa, I disable the buildings supposed to be in the second menu whenever it is not used. This might require you to do some trickery with the spell book and giving it the build order id. That is not what I did, but probably a lot cleaner.
Basically you have to play around with 'limit training/construction of unit' of the units of the appropriate types in each menu.