Tier 2 and working Extra hours

Hi everyone.

Please I need clarification with the following:

Im on a Tier 2 gen visa working as a nurse in a Trust.

I signed up for the Trust bank shift, but did not see/read a minimum or max hours of bank shifts I can do.

Does working for bank shifts with this same Trust (which is also my Main employer/sponsor) considered a 2nd job or as overtime?

Your inputs/advice is much appreciated. Thank you.

If you work as a bank you will receive a second payslip which means it's a second job, so 20 hours maximum.