If you have the dual wielder feat, and you are riding a mount can you wield two lances. Also what would the damage dice be?
Yes, you can.
Also, a lance requires two hands to wield when you aren’t mounted.
So as long as you are mounted, you can use a lance in one hand. The Dual Wielder feat allows you to use two-weapon fighting with any one-handed melee weapon, so this works fine.
As for the damage die, the damage die for the lance is a d12, and there's nothing in its special property or in the dual-wielding rules that would change this.