If you work for two employers, you're allowed to exercise you rights regarding maternity pay and leave with both of them, on a pro-rata basis for each job. However, you must ensure that you have worked for each employer for 26 by the qualifying week (15th week before the baby's due), and that you give the correct notification of when your maternity leave with each one will start.
In total you should receive the same amount of maternity pay as someone who works in a full time job. If you have worked for one employer for 26 weeks by the qualifying week, but not the other, you will only be entitled to receive maternity pay for the first one.