Taking on additional work with another employer

Hello,
I would appreciate your advice.

I have received my new skilled worker health and care visa this month and started job with my new Employer in the Private hospital UK.

My contracted hours per week are 40 hours on basic rate 10.10 and on SOC 6145.
I wanted to know whether I can take work in NHS hospital in Operating theatres and ICU as a Theatre Practitioner (SOC is different) Pay is higher than 10.10 as I have received my Registration PIN this month.
Am I allowed to work for another employer, that's the NHS.
Does the additional work need to be 20 hours or could be more ?

Thanks in advance.

Same question as above. Also this 20 hour limit is calculated and averaged over a few weeks? How many weeks?