There are few ways to do that, I'll give you a couple of them
1. Use the Father Child properties to link the form to the subform, using
the values of the combo boxes for the father and the link fields in the sub
form as the child value
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2. link the recordsource of the sub form to the two combo boxes

Select * From Inventory Where Field1= forms![MainFormName]![Combo1Name] And
Field2= Forms![MainFormName]![Combo2Name]

You need to refresh the sub form every time you change the value in the
combo, so enter the code me.SubFormName.Requery on the after update event of
each combo.
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I hope that help