The two shield cards in the game (Arrow Catching and Crabshell...apologies if there are more I've missed) both state
You cannot equip another Shield Upgrade or a Two-Handed Weapon Upgrade
The Adamatine Lance states
If you are ever without a Mount Upgrade, you must immediately discard any Shield Upgrade you have equipped
There's nothing on the mount cards (like Warhorse or Giant Vulture) stating that having a mount allows you to equip a 2-handed weapon like the Lance along with a Shield, but the Lance appears to suggest that this is possible.
Question is: Can a creature equipped with a mount also equip an Adamantine Lance or other 2-handed weapon along with a shield?