I have enabled a hidden debug menu item in PersonGlobals (Group 0x7F01EC29) named DEBUG-Force Twins which changes the # Of Babies to Have attribute of the Pregnancy Controller to 2.
Unzip the archive to your Downloads folder, When clicking on a pregnant sim you should get a menu-item DEBUG - Force Twins,

http://www.mediafire.com/file/s07h0...eTwins.zip/file