Can I short live Rc and Rh terminals together as R?

You need to return it and get a different 'stat

The thermostat you have is a refresh of the Nest "E" model, and thus is a stripped-down version that doesn't support more sophisticated systems, such as your two-transformer system, or multi-stage systems of any sort for that matter. You will need to return it and get a full-fledged 3rd generation Nest, or some other thermostat for that matter, that has separate Rh and Rc terminals.