Data Validation & Two selections from Two unique cells populating a third list

What I have is a a pretty long data validation list for selection, in which one cell's DV list is populated by another. Simple stuff. Well, I'd like to have that one cell (J5) have its data validation list populated by the result of two selected values (and have duplicates removed).

The formula I currently have for J5 is
=OFFSET(Class_List!$D$1,MATCH(J2,Class_List!$D:$D,0)-1,1,COUNTIF(Class_List!$D:$D,J2),1)

And can be seen here, as only options from J2 are included in the list.
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So I'd like to make a selection in J2 and J4, then have my J5 list include all (sans duplicates) of the yellow entries in the 2nd screenshot. Have I approached this in the wrong manner?
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*Edit* Here's my doc FE13 LAMS V_1_4.xlsm