There is a totalization agreement.
Once you are paying Swiss social taxes, you can stop paying US Social Security. That does not mean, assuming you have payed in enough quarters to qualify for US social security, that you will collect two full pensions upon retirement. If you receive benefits from a second country's pension system, the amount you receive from US SS will be reduced.
Medicare does not cover US citizens living abroad, by the way.
From American Citizens Abroad, articles on social security issues for overseas Americans:
http://www.aca.ch/joomla/index.php?o...d=35&Itemid=45
For specific questions, contact:
Federal Benefits Unit, US Embassy Bern:
http://bern.usembassy.gov/federal_benefits_unit.html