Transformers with multiple inputs are allowed in general. But variable_costs, efficiency and nominal_value of transformers are on output flows (implemented in #596).
In some cases it makes sense to have a relation for the inputs. Take an electrolyser: How much water and power is needed for 1 m³ H2?

For now we would model an electrolyser as transformer with one input (electricity) and calculate the costs of water separately. But an optimization taking into account prices for water could be asked by end-users.

How to implement:
For a start see oemof documentation for transformers, for example CHP with two inputs and two outputs.