1. 为什么不能说although...but...

没错，许多语法书和词典都说不能将 although 与 but 连用，它指的是不能按汉语中的“虽然……但是……”直译成 although...but...。比如：

虽然他很忙，但他还是来帮助我们。

误：Although he was busy, but he came to help us.

正：Although he was busy, he came to help us.

正：He was busy, but he came to help us.

上面的“误”句错在它“四不像”：因为从前面的 although 来看，它引导是让步状语从句，整个句子应该是复合句；但从后面的 but 来看，它应该连接两个简单句，构成并列句——所以，最终它既像复合句也像并列句，但实际上既不是复合句，也不是并列句，所以它错了。其实这不是惯用法的问题，而是句子结构“混了套”。

这类句子的改正办法是：去掉从属连词 although，使之成为并列句；或去掉并列连词but，使之成为复合句。

2. 你的句子是完全正确的

你的这个句子虽然出现了but although，它是一个完全正确的句子：

Soon he was President of the new South Africa. But although he had suffered for so long, he did not want to take his revenge on the white minority. 不久他成了新南非的总统，但尽管他遭受了这么久的痛苦，他却不想对少数白人报仇。

这里的 but 与 although 不能连在一起来看。其实 But 是承前转折——前面说“不久他成了新南非的总统”，后面接着有了转折，“但尽管他遭受了这么久的痛苦，他却不想对少数白人报仇”。注意，but 后面才是一个完整的含让步状语从句的复合句：

从句：although he had suffered for so long

主句：he did not want to take his revenge on the white minority

你的句子也可以这样改写一下：

Soon he was President of the new South Africa. But he did not want to take his revenge on the white minority although he had suffered for so long.

这样你就不会误认为 although 与 but 用在一起了。

下面给出两个类似的句子，你自己分析一下：

Many members hesitated but although each was pressed to act, none was in the end willing.

There were several knives in the drawer, but although each was tried in turn, none was sharp enough to cut through the rope.