I use DF 31.25 with MLP mod 1.71. Basically you play as a race of intelligent horses, with unicorns having 2 grips: mouth and horn (telekinesis). Mouth is roughly equivalent to right hand and horn to left hand.
Glossary:
When you go to m->e->squad->dwarf, you have equipment list.
When you do v->i on a dwarf, you see inventory list.
I've run into an issue, where a unicorn equipped with a crossbow, axe and shield didn't fire his crossbow, despite having quiver and bolts, so I decided to do some research.
I did a lot of tests and finally figured out backup weapons, I think. The unicorn keeps weapons in the mouth and shield with horn. He has a very strong preference for the first weapon in his inventory. Therefore the correct order for equipment list is:
1) Unforbid all weapons currently in inventory.
2) Go to squads -> equipment and remove weapons and shield. Wait until dwarf empties them from inventory, and carries then all to stockpiles (about a minute).
3) Assign: Shield, Crossbow, Battle Axe in this exact order.
The result is that he will use his crossbow and spray bolts at the enemy (Which gives him the advantage of range). Once out of ammo, or engaged, he will switch to close combat. He will use his shield, but unfortunately he will hit with his crossbow 95% of the time. The remaining 5% is what gives him an advantage over pure crossbow user.
Weapons are held in mouth and a shield is held by horn. Contrary to what some people say (maybe they talk about 0.34?) you don't need a free grip to fire a crossbow. That's why Pegasai (who only have mouth and no horn) can do it. Therefore having a shield is not a problem. The problem is that a dwarf uses the first weapon on inventory list about 95% of the time. Therefore if the dwarf equips a crossbow and an axe, what happens depends on their order. If axe is first on the inventory list, the dwarf won't fire his crossbow, and just charge into melee and use the Axe. If crossbow is first, the dwarf will fire it until he empties the quiver, then charge and use the crossbow as melee weapon. After a few pages of hitting the enemy with it, Axe will occasionally be used. Another problem with this setup is upgrading quality: A dwarf who picked up a -crossbow- and an =Axe=, then upgraded the +crossbow+, puts it at the end of his inventory list. This results in going melee with the axe.
I also found out that firing 25 wooden bolts from -crossbow- by a newb misses most of the time, barely softens the enemy up and generates 30*25 EXP for both crossbow and archer skills. With a masterwork crossbow, my test subject was able to hit much more often, despite being a newb. Silver crossbow may help a bit, but I got pages of 'Pony hits Animal with -steel crossbow-, damaging the skull, so I doubt silver will help much. Emptying a quiver at an archery target granted only 150 EXP to those skills, 5 times less.
In one 31.25 fort, I tried dual-wielding two weapons and two shields, but it resulted in weird bugs with dwarves having problems equipping terminator armour.
TL;DR: The weapon higher on the inventory list is almost always used, unless forbidden. When a dwarf upgrades equipment, the new item is at the bottom of their inventory list. You don't need a free grip to fire a crossbow. The equipment list is important in determining which items are obtained first and are therefore lower on the inventory list.
Can anyone confirm or correct my findings?