Binding to more than one property is essential in many situations. The fact that it wasn't included in the first version is pathetic. And no, using a parameter is not sufficient.
Beginning with the Anniversary Edition we have introduced ‘function bindings’ in x:Bind which will enable multi binding scenarios to be completed. x:Bind has performance improvements over the Binding markup extensions and we have been investing in that area. You can mix and match Binding and x:Bind capabilities as well as you transition to x:Bind.
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