I'm intrigued by, but have no personal experience in giving a dwarf both a crossbow and a melee weapon.
A combination of a crossbow and a shortsword would be a lethal combination.
Currently I just go with massed crossbows. 60+ marksdwarves standing around in a huge blob of pointy death is enough to handle anything. The few pincushioned survivors of the barrage pose no threat, and are clubbed to death at their leisure.
The nice thing about massed marksdwarves is that they don't need to be very skilled. On a flat surface, bolts will continue traveling. This means if they miss their intended target, they will hit the target behind or next to the intended target.
If you have a siege advancing through a narrow corridor with a blob of marksdwarves at the end, its a shooting gallery. Every bolt will hit something. Due to bolts hitting the first thing the bolt reaches, this means that the goblins in front will take hits.
After the goblin in front is hit and drops, wounded, the goblin behind him will step forward and then also be hit. This works out in such a way that everyone will be severely wounded and unable to fight back as each rank of goblins steps forward to get bolts to the face.
Likewise, this also works for your marksdwarves. Wounded marksdwarves will drop unconscious, and the still conscious marksdwarves will step forward, acting as a literal meatshield to their wounded comrades. At the end of a siege its common to have a large number of minor injuries, but actual deaths are extremely rare. Give them shields, high quality helmets, and breastplates and chainmail to protect their vital organs. An arrow to the arm is far more easily treatable than an arrow to the brain.