If you have 2 jobs and you fulfil the criteria for both (you've been working for them long enough etc.) you are entitled to 2 sets of SMP. I know it sounds unlikely but I have 2 jobs and they both told me I would get SMP. I called payroll at my second job (which is working for the city council) and apologised that I hadn't made it clear that I would get SMP from my main employer but they insisted I was entitled to it from them aswell as from my main employer. I've done a lot of research on this since then as I don't want to think I have all this money and then have it taken away from me - 39 weeks at £117.18 is over £4500!!! - but it appears to be true. Also, you can take maternity leave at different times - so I can return to my main job but still get SMP from my second job. It's a crazy loophole but it is there. Have a look at the government's website for back up on what I'm saying.
http://www.dwp.gov.uk/lifeevent/benefit ... p#whatknow
The only thing to remember would be that you only get one set of personal allowance so one set of SMP would be taxed at the full amount.