Is it possible for LTM4637 to generate negative output? eg:12V input -5v output 20A?
Yes, LTM4637 can give negative output. But for negative Vout config, the inductor current is not exactly the same as inductor current, so it may not give out 20A as in buck config.
Yes, you can parallel two LTM4637. You can also use one LTM4630A. LTM4630A is a two phase controller. Two phases in parallel can give 36A in total.