Question is: Do 2 10A fuses in parallel make  1 20A fuse?
I would have thought not as both fuses wouldn't be exactly the same spec and so path of least resistance would be taken.

I could be wrong... I could be right as Mr Lydon would say

Then the one drawing more current heats up more, slightly increasing its resistance.

 I can imagine some kind of feedback loop happening.

Would never not blow over its 20 amp rating, but could also certainly blow under it.

Its a safety, safety fuse.