If you want to fly a spinnaker from the same elongated boom as the self-tacking jib, you would have to furl the jib and lock the boom amidships. One way of doing this is to hook a strop under the aft end of the boom and tie it to a cleat on the front of the mast. I think Alf devised a way of tying it down to the foredeck instead. You might also need to rig a bobstay from the front of the boom down to the towing eye on the stem, to stop the front end from waggling about too much.
Attached a photo of my (now abandoned) experiment with a longer jib boom, made from an old carbon spar. In the end, I opted for a fixed but removeable plank bowsprit, off which I can fly any one of the following: large conventional jib, flying jib, code zero or asymmetric. I usually combine the front sail with a self-tacking jib on its own jib boom, otherwise there is a bit too much string for a solo sailor to deal with all at once. Apparently some old sailing barges combined a self-tacking jib with a flying jib on a fixed bowsprit, and they knew a thing or two about sailing short-handed.
