Hi. I think the info you want is here:
http://www.dwp.gov.uk/advisers/ni17a/ma/ma_5.aspThe relevant paragraph is the one headed "Claiming if you still work for an employer"
Looks like you need to give your matb1 form to both employers so they can decide if you qualify for SMP, then if not they'll give you a form to say so which you need to send with your claim for MA.
It reckons the earliest you can get your matb1 is after the 20-week scan.
HTH!