From xperthr:
If a pregnant employee has more than one job, whether or not she is entitled to statutory maternity pay (SMP) for each job depends on how the Class 1 national insurance contributions on her earnings are worked out.
If all of the employee's earnings have to be added together to work out Class 1 national insurance contributions, they also have to be added together to work out her average weekly earnings, so she is entitled to receive only one lot of SMP. This is usually the case, and indeed is the general rule, where the employee has two or more jobs with the same employer or with associated employers. The only exception is where it is not reasonably practicable for the employer to add together the earnings from the separate jobs. The onus is on the employer to show that aggregation is not reasonably practicable, taking into account the costs, resources and effects on running the business. Employers are considered to be carrying on business in association with each other for these purposes if their respective businesses serve a common purpose and, to a significant degree, share such things as accommodation, personnel, equipment or customers.
If the Class 1 national insurance contributions on the employee's earnings are worked out separately, the employee's average weekly earnings must also be worked out separately. As a result, the employee can receive more than one lot of SMP, assuming of course that she fulfils the other qualifying conditions for SMP. If the employee has two or more jobs with different employers, then the general rule is that each employer must work out national insurance contributions on the earnings that it pays the employee and should ignore the payments made to the employee in her other job. This means that if the employee works for two different employers that are not carrying on business in association with each other, she will be eligible to receive SMP from each employer, subject to fulfilment in each case of the SMP qualifying conditions.