In reply to spat:
I’m guessing that your data_0 and data_1 ports are declared as ‘inout’. In this case, you can’t connect logic data types to it. Instead, you need to use the wires like you had originally declared.
My approach is to create all connections in an interface as wire, and also create related register variables. I will then create a set of assign statements to the wires based on the output enables.