After finding the right search terms (concatenating hash functions), I found a few stackexchange discussions about this, which lead me to other methods like truncating a stronger hash function[1], and chaining hash functions[2]. Apparently TLS already concatenates MD5 and SHA1 [2][3].
Given that the article is about collision attacks and not preimage resistance, that was my main thought when thinking of the issue. I'll leave it to the experts to figure out what's the best for cryptographic hash functions.
> Apparently TLS already concatenates MD5 and SHA1 [2][3].
Luckily TLS no longer does such things. This was a bad workaround in old versions of TLS. Which they then replaced by a "you can use secure or insecure hash functions, you choose" in TLS 1.2 (which is hard to excuse - at the time TLS 1.2 was written the weaknesses in SHA1 and MD5 were well known). In TLS 1.3 finally they did the right thing and only support secure hash functions.
Luckily TLS no longer does such things. This was a bad workaround in old versions of TLS. Which they then replaced by a "you can use secure or insecure hash functions, you choose" in TLS 1.2 (which is hard to excuse - at the time TLS 1.2 was written the weaknesses in SHA1 and MD5 were well known). In TLS 1.3 finally they did the right thing and only support secure hash functions.