I've done a bit of reading about UK employment law, and I have some good news.
(1) A night worker’s normal hours of work in any reference period which is applicable in his case shall not exceed an average of eight hours for each 24 hours.
(2) An employer shall take all reasonable steps, in keeping with the need to protect the health and safety or workers, to ensure that the limit specified in paragraph (1) is complied with in the case of each night worker employed by him.
The question is, what is considered "all reasonable steps".
If you have a rostered job on Saturday that goes for 8 hours, and your other employer wants to roster you on so that within that 24 hour period you would be working for more than 8 hours, then it would be very easily argued they are not taking reasonable steps. It would be very different if you were trying to hide your other employment from them. But from what I can gather, you are not.
Point 2 is deliberately soft. If an employer ONLY had to be concerned about employment at the same organisation, the language would be a lot more direct. But it is soft in recognition that it may not be possible for the employer to know, for instance, if there is a second job.
I tried to find supporting interpretations on the internet. Regarding the maximum working hours per week (48 hours), I found quotes from a few different employment experts:
The Regulations state that an employer must take all reasonable steps to ensure that the 48-hour limit is adhered to. Organisations that actively ignore other work undertaken by its staff will not meet this obligation, but this wouldn’t necessarily apply when the worker deliberately misleads the employer about what else they do.
Where there are weeks that employees must work in excess of 48 hours, employers must also be vigilant in making sure that the requisite rest periods are observed.
There is no issue with an employee working a second job, although employers must take all reasonable care in order to adhere to the above obligations.
Ultimately, you should provide both employers with documentation around when you work all shifts.
Now that your Mon/Tue employer is aware of your Saturday shift, they must take reasonable steps to ensure there is no breach of the act. Note that telling you to not work the other shift would probably not be considered a reasonable step.