Cadel,
That depends on the publisher. Black Lace has extensive submissions guidelines with lots of tips and suggestions http://nt6744.vs.netbenefit.co.uk/guidelines.html ( Check out their comments on the SM theme) but my quick read through didn't catch any prohibition against duplicate submissions. God, does that sound sexy or what? Submit girl, again submit....
Anyway did I mention the role of patience; publishers take a long time to read, judge, edit, and publish a book. This is doubly true for print media. When they make the commitment to publish your work they almost always will require exclusive publishing rights for some extended time period, some may even want permanent publishing rights of the copyrighted material you have submitted. Trying to play one publisher off against another just doesn't work unless you are really well known, someone like a Clancy, King , or maybe Wally Lamb.
So my suggestion is, unless they specifically say multiple submissions are OK just pick one. Send it out, keep polishing your work as you wait for the rejection slip, then send out a new proposal letter to the next publisher on your list. Steven King collected a lot of rejection slips before he found someone to publish "Salem's Lot" and he had an agent. Don't give up, the publishers need writers just as desperately as writers need publishers, maybe more so.
Best of luck
Mad