I don't mean related at all to TLS. I mean in general, is the principle of using multiple algorithms that bad? If, in an alternative universe certs were signed

I don't mean related at all to TLS. I mean in general, is the principle of using multiple algorithms that bad? If, in an alternative universe certs were signed with MD5 and SHA1, would that not have been a practical defense to buy some more time? And thus for new, incompatible protocols, is it not a safer choice to use a couple of algorithms? That way if SHA2 or 3 are found to have some terrible weakness, you're still OK. And if practical attacks are found on both, it still might not mean that such attacks work together? There's a bit of precedent in 3DES which just runs the same algorithm 3 times in a row... Which I guess you could do by using the same hash algorithm several times on the same input plus some fixed data or transform. So even though you can find two inputs a and b that h(a) = h(b), you might not be able to get h(f(a)) = h(f(b)) hmm? (Though I understand certain hash attacks will do exactly that for certain values of f.)