Not exactly.
If you have an uber jewel in equipped gear on your player, you cannot socket it into equipped gear on your player
If you have an uber jewel in equipped gear on your mercenary, you cannot socket it into equipped gear on your mercenary
You can't socket an uber jewel into an item that already has another uber jewel in it
You can't equip an item with an uber jewel if you are already wearing another item with an uber jewel in it
You can't equip an item to your mercenary with an uber jewel if your mercenary is already wearing another item with an uber jewel in it
Limiting you to 2x uber jewels per char, one on your player, one on your merc