Surely the two sections would need to have the same number of poles
A two-pole synchronous motor will run at 3000 rpm on 50Hz. A two-pole induction motor might manage 2800, 2900 with the wind behind it, but there has to be some slip or else there would be no induction. And surely you would want the induction section to take the motor as near as possible to synchronous speed for reliable handover to the synchronous section? That would require the same pole count; a 4-pole induction motor would only manage 1400-odd rpm, not enough for a 2-pole synchronous section to catch up from.