We have implemented the Operand Type button, which allows end-users to select an operand type: a value or another field's value. By default, this button is not displayed and can be shown by the ShowOperandTypeButton property.
For a filter control embedded to the ASPxGridView and ASPxCardView controls, we have implemented the ASPxGridViewFilterControlSettings.ShowOperandTypeButton and ASPxCardViewFilterControlSettings.ShowOperandTypeButton properties.
You can try this functionality in the following Code Central example: How to construct rules like [Column1] [operation] [Column2] in ASPxGridView FilterBuilder
Note that the specified links will be available with the next major release 15.2.