On easy I accidentally got it after the first election. You just need SPD + DDP + KPD to be 50% (or above 50% I have not tested this) but you don't actually have to be in coalition with them, as long as you are in government. On normal I suspect you need to get the Weimar coalition then investigate corruption or pass land reform or something ASAP, get an election and win that, then go into coalition with DDP and Z again.