Multiple Boarding Parties
If two or more allied boarding parties board the same starship, they can choose either to operate independently or combine forces. When operating independently, resolve each boarding party’s boarding attack separately.
When boarding parties combine forces, add their number of combatants together to calculate their complement bonus, and use the average of their training modifiers (rounded down). Combine the boarding parties’ combatants to determine how many combatants are incapacitated during the boarding event.
Source
Starship Operations Manual pg. 41