beside #2602 using the example from the demo (changing HorizontalContentAlignment to left or right) but changing the second header to add more text will result in both tabs to have the second tab width (the widest tab)
<TabControl HorizontalContentAlignment="Left" materialDesign:ColorZoneAssist.Mode="SecondaryMid"> <TabItem Header="TAB1"> <TextBlock Margin="8" Text="Full-width secondary example tab 1" /> </TabItem> <TabItem Header="TABBBBBBBBBBBBBBBBBBBB 2"> <TextBlock Margin="8" Text="Full-width secondary example tab 2" /> </TabItem> </TabControl>

also it's doesn't handle overflow