You should be able to get two on there. We do that on my dad's car all the time.

The inner diameter of the eye-bolt is so narrow you can't get the second harness clip fully seated and locked. Well, that's not entirely true. If the hook is totally exposed (as on the harness bar), you can do it with some amount of creativity, but that's not possible in a situation where the access to the bolts is limited by the geometry of the chassis. I spent a good 15 minutes trying to convince two to go in together and it just wouldn't. Perhaps there are bigger hooks than the ones I got. This is the only type the local race shop had.

I'm Black91's dad, and it's my car he's describing. For the shoulder belts, I clip both of them onto a single eye bolt on the harness tab right behind the seat. I suspect they are exactly the same eye bolts you are using (I've only ever seen one design, from a variety of suppliers). However, the clips sit completely flat and are leaving in opposite directions. (the eye bolt is parallel to the long axis of the car, the shoulder belts are heading slightly left and right). It is easy to get them to clip on, but there is only one order in which to put them on that will work, as the latch portions can interfere with one another otherwise.

In the case to the lap belt/ sub belt, I suspect you'd have problems. You are trying to have the belts depart in more or less the same direction. I'd suggest you do the subs bolt in, using the eye bolt to bolt them in. The subs could just live under the seat, and unclip the laps when you're not using them.