As far as I can tell there is no keyboard shortcut to select both characters, that is definitely needed.
You can right click on weapons/armor and give them to your other character, however you cannot do this with all items in the inventory. I have no idea why this would be the case. I wanted to give certain agility/constitution potions to my other char and had to drop it on the ground first to do it. Although I guess you can drag it to the portrait as stated above (haven't tested that). Also you cannot right click->give to gold either. This would all be eliminated if both characters shared an inventory like they share experience pts.
