I have two 52" ceiling fans on one circuit hanging from a vaulted ceiling. With blades pushing up (winter) low speed still creates a draft during cold periods. Can an off-the shelf control lower the speed on two fans, or is the rpm determined by the windings?

Following is a link to a fan speed control that can handle 5 Amps of fan motor(s) that may be large enough:

http://www.lutron.com/instructions/033039c.pdf

The fan control should exceed the amp load of the fans plus 25% . If you don't know how much your fans draw, check some of the fans at the store you purchase the speed control from to assist in your calculations.

You will need to leave both fans on high with the builtin switches, and control with the speed control.

A proper wall mounted speed control should solve your problem. The comments about ensuring the proper amperage ratings are good ones, however if you're using one control per fan, any 3 or 4 speed control should be fine. If you are using one control for both fans you will need to look for one with a higher amperage rating. Keep in mind only higher quality fans with 18 pole motors can use solid state infinitely variable speed controls, such as the Lutron mentioned.

For more information on ceiling fan controls (and more):

http://www.ceiling-fans-n-more.com/ceiling-fan-faq.php