Here is a good question: upload the same stuff to Getty and iStock as well? Or just to one of them? I know Thinkstock sells some iStock stuff as well. Is Getty and iStock selling the same library or they are completely different?
Like Sean said, you can't upload the same stuff to both.
However, there is an overlap, as some iStock exclusive images are available from Getty.
Also the premium access buyers have access to Getty, iStock and Thinkstock.
(To save you the bother of asking, if you google an iStock image which isn't officially on Getty, and go to it in surprise, you'll see it has a message with a fake reason why it can't be bought, but without any link to buy it on iStock. Insanity.)