Python script, checkbox NumAcross not working

I can't seem to get the checkbox NumAcross option to work in my python script. Here is the popup, even with NumAcross set to 2 they show up in a single column. Anyone have any ideas? The scripting reference doesn't show the syntax for this in python but my other popups with dropdowns work fine with this syntax.

Code: Select all

uirotate = {
    1: {1: 'rotdir', 'Name': 'Rotation Direction: ', 2: 'Dropdown', 'Options': ['CW', 'CCW'], 'Default': 0},
    2: {1: 'c1', 'Name': 'test1', 2: 'Checkbox', 'NumAcross': 2, 'Default': 0},
    3: {1: 'c2', 'Name': 'test2', 2: 'Checkbox', 'NumAcross': 2, 'Default': 0}
}