Ilan,
As a senior engineer, you just delegate the task of designing and implementing a two-phase flow meter to Reuven. I guarantee it will get done, and very interestingly.
I like Kevin's approach. If you can do careful flow tests on the wells, with James tubes and weir boxes, you can generate curves for steam and brine flow vs wellhead pressure. Then, when the wells are in service, the curves can be used to reasonably approximate the operational steam and brine flows.
There are orifice plate correlations for two-phase flow, but as was previously mentioned they are very dependent on flow regime. Also, to get reasonable results, the pressure drop across the orifice must be fairly high, more than 20 psi, which is not good for the well production. I don't like any orifice plate in a two-phase line for that reason alone. Millions are spent on the well and piping system in order to get the fluid to the plant and it doesn't make sense to me to put any intentional restriction in the pipeline.
I like Kevin's dual tracer concept also, but have never used it.