The original Hobie 16 mast had a single-hole tang, like the Hobie 14. The idea was to keep it simple and use similar, if not identical parts between the two boats in production at that time.
However, the original 14 did not have a trapeze, and most people ended up putting a second trapeze on the 16. While possible, this put a lot of stress on the one hole mast tang. The hole would elongate and eventually break out - which would be a really bad day on the water.
In the late '70s the Hobie 16 mast tang was changed to the three-hole tang to spread the stresses out. Standing rigging on the lowest hole; trapezes on the upper smallest hole (the biggest hole is for the body of the trapeze shackle to pass through - it doesn't have anything attached to it).
So, the short answer to your question (can a single-hole tang support a double trapeze?) is, yes, but not forever. You need to check the tang frequently for wear.
You don't need an oversized shackle to add a double trapeze. One solution is to put the second trap wires on the pin of the shackle, like this:
(That's a 14's rig, but the principle is the same.)
The other way to do it is to have the trapeze wires be continuous - the skipper/crew wires on one side are the same wire, with only one thimble at the mast tang.