WPF share column width between separate grids

It is possible by using SharedSizeGroup. Also check out IsSharedSizeScope.

<GroupBox Grid.IsSharedSizeScope="True">
  <Grid>
    ...
    <Grid>
      <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" SharedSizeGroup="A" />

<GroupBox>
  <Grid>
    <Grid>
      <Grid.ColumnDefinitions>
        <ColumnDefinition SharedSizeGroup="A" />

See here for more information.