Is the only characteristic of a twin fin that separates it from a fish the fact that it has a more shallow swallow tail and a widepoint that is more towards the middle? Does fin placement and width matter?
Only if the sky is blue where you live. Otherwise try a submersable skateboard.
There are a few differences between a twin and a fish. Comparing templates, a twin has a slightly narrower nose and tail width. A lot of times a twin will have wings with swallow tails averaging around 5" tip to tip, compared to a fish which usually has straight swallow arond 10" or more from tip to tip. Most fishes have flat bottoms, where as a twin will have a panel vee in the last third of the bottom. The twin will have regular shaped fins, toed and canted, where a fish has keel shaped fins with long bases of up to 8 or 9 inches, that are set paralell to the stringer, with little or no cant or toe in.
I had an MTB twin that was a winged pin with channels…ridden to death and now templated and roughly duplicated twice or more by local LI shaper Keith.