Instant start lamps tie both leads together and you can do this either way you mention. You could twist together the two leads in your left picture and it would be electrically identical to the one in the middle picture.
Your ballast is designed for four bulbs. Is that what your exposure unit will have? Take a look at the wiring diagram at the bottom of the following web site and tell us if there is anything that is different than your intended application, or if there is anything you don't understand. It is not a good idea to run a ballaast with fewer bulbs than it is designed for.
One thing that is not always obvious about Fluorescent lamp fixtures is that the bulbs need to be positioned next to a metal plate that is held at ground potential. If you look closely at most Fluorescent lamps you will find that they always have metal structure directly behind the bulbs. If you look at the wiring diagram I just referenced you will notice that they tie this metal structure to the green ground wire coming from the fuse box. This grounded structure provides a ground potential for the bulbs to fire against. Your exposure unit needs to provide this ground plane either by using a commercial lamp fixture or by you in your design. You could try getting by without it, but it may, or may not, work and at best it may prove to be unreliable.
The reason for the two leads coming out of each tombstone, as you call it, is that in non-instant start applications, the bulbs contain a small filament winding across these two terminals at each end of the bulb. These filament windings heat up and make it easier for the non-instant start ballasts to get the bulbs to fire across the length of the tube. After the bulbs fire, these starter filaments do nothing usuful and just set there sucking up power. Instant start ballasts solve this problem another way. They have the ability to generate a very high voltage at power on and this high voltage can fire the bulb without the need for the hot filaments. Therefore, in an instant start ballast application they just tie the two filament leads together since they don't need to run a current through them to warm up the filiment winding.