Trams that can be used with both high platforms and on street sections are perfactly doable, the trams in San Francisco have got steps that for the high platform sections of the routes raise up to form a level platform level with the main floor of the tram
Probably impossible today with DDA/RVAR requirements, and undesirable from a complexity and reliability perspective anyway.
Manchester Metrolink started with a similar arrangement. Vehicles with high-floor passenger saloons stopped at new low platforms in city centre streets, accessed by folding vehicle steps. High platforms were retained on the former heavy rail lines to Altrincham and Bury to save significant work either raising the trackbeds through the old stations (often difficult due to overbridges nearby) or cutting down the platforms. To maintain disabled access through the centre, each low floor stop had a short high platform section sufficient to allow level access for one door with ramps or lifts and stairs providing the transition to pavement level. Later, Manchester rebuilt all the city centre stops with high platforms throughout, and that also became the standard for all subsequent extensions. The new fleet is thus not equipped to call at low platforms at all, saving the complexity of all those folding step mechanisms.
Conceptually, tram-trains can be high or low floor. If low floor trams inter-run with high floor heavy rail vehicles, split level platforms can be employed at any common stations, as proposed for Sheffield's Rotherham extension, or if a new network is proposed with relatively few new light rail stops and perhaps very little traditional street running, then high floor vehicles and platforms could be used. High floor light rail systems like Manchester are quite common in Germany, although these are not (currently) the same systems employing tram-trains.