Elon said in a Q&A in Norway that they do plan to increase the Supercharger power delivered to cars to 120 kW.
As for power sharing, if two empty cars plug in, they'll both get half - 60 kW. But normally the cars wouldn't arrive at the same time or with the same level of charge. The charge rate is tapered off once the car is half full. So once one car starts ramping down, the Supercharger will reallocate that power to the other car, and it will start ramping up (until it needs to taper off too, of course).
Sounds like they have a very elegant architecture inside the Supercharger. The individual chargers inside the Supercharger stack obviously can be allocated to either car as needed.
Here's a tip for Supercharger users: if you arrive at a location with more than two slots, try to avoid parking beside another car! That way you won't end up sharing a Supercharger with it, and will probably get more power.
