It depends.
If you work for two companies that are unrelated and unaffiliated, you can make
salary deferral contributions of up to $20,500 between the two plans with no more than $13,000 going to one of the
SIMPLE plans in 2008.
For this purpose, unrelated and unaffiliated means that the companies have nothing to do with each other, such as having the same owners and sharing resources.
If the companies are related or affiliated, your total salary deferral contributions are $13,000 (for 2008), which can be split between the two plans.
For more insight, check out the
SIMPLE IRAs Tutorial.
This question was answered by
Denise Appleby.