CAN YOU WORK FOR 2 COMPANIES AT ONCE?

Are you allowed to work for 2 retail companies at once. I have recently gained a job at next and start next week and i am happy about this. But I have got an interview at River Island tomorrow which is the job which i have wanted since the begining. if i was to get the job at riverisland am i allowed to leave Next even though i have only just started? And should I let river island that I already work at next, Will this deduct my chances at gaining the job.....PLEASE HELP...ThankYou