I took a look at this and I can't see how you can create a new two colour gradient in Excel 2011 for Mac. You can open a spreadsheet created in Excel 2007 for Windows, with cells with this type of two colour gradient in Excel 2011 for Mac and the cells display the two colour gradients fine. You can even copy the format to other cells, but it looks like Excel 2011 does not have the ability to create new two colour gradients.