How do I double underline in numbers on the iPad?

From the "Format" menu (not the toolbar) select the "Text" submenu and the "Show Fonts" item (or type Command – T). This opens the font panel.

Clicking on the underline button brings up a menu with choices for none, single, double and colored underlining (color is used in combination with either single or double).

You're right, of course. In my defense, I was replying to a question that showed up in the Numbers for Mac discussion area (although now it is, correctly, in the iWork for iOS discussion).

I suspect you can only create this styling in the full, Mac version of Numbers. In a quick test on my iPod, it appears that if you format a number with a double underscore on a Mac and then import that spreadsheet to the iOS version it does preserve that formatting and you can then copy and paste that cell (to the same or a different spreadsheet) and edit its contents without losing the formatting. I don't know my way around the iOS version too well, but I don't see anything comparable to the "Copy style" and "Paste style" commands in the full version of the application.

This is a lot of hoops to jump through to get this formatting, especially if you don't have a Mac, but if it is really important to you to get this in your iPad document and you don't have access to the Mac version to create it, I have a small spreadsheet which contains a couple cells formatted with a double underscore, here: http://db.tt/Nu27GwDw which you could copy and paste into your spreadsheet.

The font panel (Format – Text – Show Fonts) has controls for double underlines and colored underlines. This is a system feature, available in most Cocoa applications.