Wow, you've gotten a lot of good advice...but I have to say that no matter what is "normal", your situation is causing trouble for the two of you and that is NOT okay! My husband is 5 years younger than I am...needless to say he is often more interested in sex than I am. Honestly, I finally had to have a talk with myself. I had a great husband who couldn't keep his hands off of me, and who thought I was gorgeous having just had a baby, yet I would get annoyed when he would interrupt my mom duties and think about sex all the time! I slowly started reminding myself that I am happy that he wants me often, even if it was just forcing a little smile to let him know that I was NOT pushing him away. Could it be that your husband feels rejected, so he thinks that if he doesn't ask for it all the time, then he wouldn't even get the twice a week? Maybe if the two of you really enjoyed those 2 times, and it was a willing and mutual time together (even 15 minutes) then he would be able to relax a little b/c he would know that you still wanted him, too. I understand your work schedules are crazy, my husband works ALOT, and my youngest is 6 months old. We actually get together every other day, at the very least. I got some good advice, once, from a woman who thought that we OWED our husbands good sex...can you believe that????? Anyway, she said that even if you have to pretend to enjoy it, it IS a relatively short time, but something great will happen and you actually WILL start to enjoy it, and your husband will think you are the greatest b/c you still want him after everything else that consumes your time.
All that said, please don't think that I am trying to turn your issues back around on you...I completely understand where you are coming from and I do not mean to fault you or tell you that its all up to you. I'm just telling you that I've been there and this has worked for me. Like I said, my hubby and I are very 'active' but it took some work. Your kids cannot have a solid foundation if there are cracks in your marriage. They are like little radars and they will detect the smallest instability. They will soak up all the time you will give them...but they will do just fine with 15 minutes less, especially if it brings mommy and daddy some closeness back.
Good luck!