In short: you accept a higher failure rate (twice as many pumps) to get a lower explosive failure rate (much less explosive pumps). A very good tradeoff when bu

In short: you accept a higher failure rate (twice as many pumps) to get a lower explosive failure rate (much less explosive pumps). A very good tradeoff when building a rocket with a high number of engines and some redundancy which does not help against explosions. It gets even better when the pumps turn out to be more reliable individually due to relative simplicity.