1 Can I use two crowwbow hand, one in each hand?
2 Can I use a crossbow and a shield?
3 Can I use a crossbow and a short sword, one in each hand?
4 Can I reload a hand crossbow carrying a weapon or a shield in the other hand.
Independently on what the rules say about
attacking in the same round (i.e. "two-weapon fighting"), I'd rather use common sense about general
use of these, before taking any feat or special ability into account.
By "use" I just mean "attack" with that weapon normally. My interpretation is:
1) Yes
2) Yes
3) Yes
4) No
Then I'll leave it to the others' comments on how you can in case 1) and 3) get two attacks in the same round, but at least if you have Extra Attacks it should be obvious that you can use both in the same round.
All these provided the crossbows are already loaded. Unless a particular crossbow says otherwise (but I don't remember any automatic loading xbows in 5e), I rule that you do need 2 hands to load a crossbow.
There is no shortcut to this, but at the same time 5e goes quite easy with moving weapons between your hands or dropping and picking them up again, so then I won't go hard on a PC in normal conditions, and will let her put down the other weapon to load the crossbow and then pick it up again. But in
not normal conditions it's different: if you are using one hand to hold on to a ledge or ladder, if you are
flying or other weird situation, I might rule you can't reload that easily.
I am undecided about the shield. Generally, I think shields should be securely strapped to be effective, and then your hand is busy and cannot be used for precision tasks. Bucklers used to allow use of that hand at the cost of temporarily not benefitting from the AC bonus, but sadly IIRC they are gone in 5e. I don't think I'd easily allow to drop the shield + use the hand to reload + strap the shield back up, all in a single turn. If this makes crossbow+shield a bad combo, so be it.