a) If I could have used a bat with two hands, I could have killed the first one surely.    

b) If I had used a bat with two hands, I could have killed the first one surely.

两句都对，意思略有差别。

第一句比第二句，多了一层意思“能够”。could 表示“能力”。

谁告诉过你，if 从句不能用 could？用might 也可以。

if 从句中，had used 和 could have used  分别表示“过去未做到” 和 “过去未能做到”。后者比前者多了一个“能够”。其他都一样。

我再用 could 举一个例子：

If he could come, we would not go out.

如果他可能来的话，我们就不走了。

意思是：他来的可能性极小。这是与“将来” 不符的虚拟语气。

以下说法，都可以：

If he came, we would not go out.

If he could come, we would not go out.

If he might come, we would not go out.

If he should come, we would not go out.

If he were to come, we would not go out.