As Sebi says, you need a two way splitter - it's also VITALLY IMPORTANT that's it's a specific satellite one! (different frequencies and power pass through).
To make life even more complicated, a LOT depends on your exact system - in the UK almost all satellite systems use Universal LNB's, these have four different settings (H, V, High, Low), switched by the voltage up the cable, and by a 22KHz switching signal superimposed on the voltage.
If you connect a splitter to this type of sytem it means that both receivers can only use the same type of LNB setting - this is EXTREMELY limiting.
If you want to do it correctly, you should buy and fit a twin LNB, this has two leads direct from the LNB to the two receivers, and allows both receivers full access. Quad LNB's are common as well, where you can have four receivers connected (or two PVR's).