In order to calculate combined velocity, you need to know the total amount of time that each team is spending on the story, as well as how much work they are producing per hour. Then, you can divide this number by the number of hours in a day, and multiply this figure by 100 to get your combined velocity.

Hopefully the Story is divided into multiple demonstrable Outputs and then to specific Tasks/Features. If this is the case, then based on the worklog done & completion achieved on the specific Task/Features by the assigned team-member, his/her velocity could be calculated from these. Hence grouping all the team members outputs completion (tasks/features), Team’s velocity could be calculated i.e., numbers of work/task/features completed in a sprint/milestone/release.

Thanks
Sandeep