How long have they both been fixed? In a male dog, it can take up to a year for all the testosterone to leave his system. So, if you just got them fixed it might take awhile before he stop.
If he has been fixed for awhile, he probably has developed a habit for marking and it is extremely hard to break. You have to catch him in the act and correct him. If you don't he will not understand why he is being corrected. Unfortunately, sometimes even that doesn't work.
However, I will give you a quick solution while you are working on his marking problem. The solution is belly bands. It basically a fabric belt that your male dogs wear to cover their private parts. When he lifts up his leg to mark, he will pee on the belly band and not on your curtains. Here is a link to what it looks like
http://mall.cairnrescue.com/inv_bellyband.htm
Good luck!