Is it possible to reroll 2 different stats on an item? Or even all stats on an item?
Once you've used Enchanting to reroll a stat on an item, that property "slot" is the only stat that can be changed on that item. For example, if you rerolled +400 Vitality and replaced it with +5% Chance to Crit, the crit chance property is the only one you'll be allowed to reroll.
Note that while there is (apparently) no limit to how many times you can reroll a given property, the gold cost goes up each time (material costs stay the same).
Note also that enchanting an item binds it to your account. This can be relevant for nice yellows and for legendaries that dropped in multiplayer games and were still tradeable.
No, its only possible to reroll 1 of the existing affixes on an item. Once you choose to do that, you can only reroll that one affix and nothing else on that item.
There are also some limitations that I covered in this other answer.