The Slingbox only only has one HDMI input, and thus, only supports one device hooked up to it through HDMI.
Do you have a TV where you would be viewing the Slingbox? Unlike your home cable service, which you can't bring with you, and may need a Slingbox for, your Apple TV is something you should be able to bring with you, where you can access all of the Apple TV content there that you would be able to with Apple TV at home. I guess what I am asking is: What are you looking to accomplish when you are away? There might be a simpler solution than trying to hook up multiple HDMI devices to the Slingbox.