Unable to get multibinding working

[francotiveron]

This works

<local:DataItemUI 
x:Name="SetPosition" 
EditOrderNext="{Binding ElementName=LoadOffs, Mode=OneWay}" 
/>

This also works

<local:DataItemUI 
x:Name="SetPosition" 
EditOrderNext="{Binding ElementName=RotationsPerFoot, Mode=OneWay}" 
/>

Now I want to bind the EditOrderNext conditionally, but none of the following work (binding returns null)

<local:DataItemUI 
x:Name="SetPosition" 
EditOrderNext="{qc:MultiBinding  '$P0 ? $P1 : $P2', P0={Binding IsGroup}, P1={Binding ElementName=SoftUp}, P2={Binding ElementName=RotationsPerFoot}}"
/>

<local:DataItemUI 
x:Name="SetPosition" 
EditOrderNext="{qc:MultiBinding '$P0 ? $P1 : $P2', P0={Binding IsGroup}, P1={Binding Source={x:Reference SoftUp}}, P2={Binding Source={x:Reference RotationsPerFoot}}}"
/>