Yes, MAME should allow you to set multiple inputs for one function. For example, Hit tab and assign the UP movement to the 4-way up, then immediately set it again for the U360 up. Two inputs should appear in the field.
So to make a long story short, you can attach the 4 way to the ipac2 (leave as is) and then just manually adjust the global settings for MAME (in the tab menu to add the u360s). You will also have to re-add the 4-way, as described above for them to both appear.
~ D