You can use the IsSharedSizeScope attached property. In your template definition, attach a "shared size group" to each column, like this:
<Grid.ColumnDefinitions>
<ColumnDefinition SharedSizeGroup="col1" />
<ColumnDefinition SharedSizeGroup="col2" />
</Grid.ColumnDefinitions>
... then define your ListBox as a shared size scope so it knows to size each "size group" the same way:
<ListBox Grid.IsSharedSizeScope="True">...</ListBox>
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