Can the splits be changed at the team level or do they have to be changed at the transaction level? For instance, if one team lead is always going to get 30% of the Lead Split?

Co-Team Leaders will by default have the Team Split divided 50/50 when the transaction is created, however it can be adjusted in the Transaction Details. 

If the second Team Leader is never going to receive commission, they should be added as a Team Admin. This provides visibility over all the transactions, but will not automatically assign commission.