VOA 原句: The most interesting part is not how much they spend, although that's important, but where they're spending it. 

这句话貌似是VOA新闻的即兴采访，也就是现场口语录制，很可能有语法错误。请老师核实。

改写后的句子：

The most interesting part is not how much they spend but where they're spending it，although that (how much they spend) is important.

Not… but…表示“不是… 而是…”。

此句中的but引导的句子作主句的表语，although引导的句子才是状语从句并且只有一个。所以我认为此句although 和but 虽然同时出现，但是没有错误。专家老师们认为呢？非常感谢！