That we cannot help you with: if you have concatenated two fields to create a new value to insert in the combobox, we have no idea where the first ends and the second begins - it's just a string now, and unless there is a very specific character between the two values, they cannot be reliably "untangled".
A much better idea would be to use this:
Flat-MultiColumn Combobox with Autocomplete[
^] or to databind the data behind it all to a single columns ComboBox and and use the DisplayMember and ValueMember properties to select the Desc_material to display to the user, and the id_material to return as the selected value.