Of course the big factor would be if your arriving flight is on time.

Immigration usually averages ten minutes I'd say, except when I ended up behind a flight from China it was more like twenty. Bags are pretty quick. So those two aren't a big factor IMO. But it's a pretty spread out airport, so if you have to change terminals after immigration, you might be pushing it a bit. If the KL flight leaves near where the ET flight arrives, I think 2 would be doable. But I'd book SQ or spend a night in the city and get the KL flight the next day, etc., just to avoid worrying about the connection the whole way to SIN.