He's dead wrong for lying. He should just tell you he's picking them up early.
Reading this thread, and one other you had posted, I think that maybe he'd like more time with his kids. But perhaps he feels he can't have that without being a little secretive about it.
No offense but my time between my daughter and I was laid out in a court room as an agreement between my ex, the court and I. Anyone else was and is secondary to that. My current wife has never been part of any agreements between my ex and I about time, or anything else I didn't ask her advise on. Her an I would communicate and then I would come to a decision about my child and work it out with my ex. At no time did my wife, previous girlfriends, etc, have any communication with my ex about when I would or would not be having time with her.
It is unfortunate that the children have to share a room together. But not as unfortunate as children not having their father's time for 50% of the time.
I applaud his ex for wanting for him to have more time with the children they share.
As parents we don't get alone time unless we have a sitter or creative scheduling like slumber parties, etc. Consider for a moment how you would get alone time vs parenting/family time if all the children were yours and his together with no ex's involved. I don't know about you, but before my divorce alone time meant we paid someone, dropped dd off at gram's or she was at a slumber party. Your husband is the father to those two children and not a sitter. His ex isn't a sitter either.
I think if H and I had 'ordinary' ex's, things would be a lot easier. Both our ex's are self centered and manipulative. H has told me many times that if his ex had her way he would be at her beck and call regarding anything (car repairs, money, kids etc.) For instance, she once told H that he had to take the kids every weekend because she was "involved with someone and trying to make the relationship work." Well, hello, H and I are, too. Another example, she once told H that her Grandma was dying, going into Hospice and she needed to be with her, so he want to get the kids for the weekend. He later found out that she went to the casino that weekend!
I agree that if the kids were all ours w/no ex's involved and we needed a night out, we would have to make plans for a sitter. I feel H's ex should do the same. She should make her personal plans for when the kids are in our care. H and I have missed out on a lot of events/invites/parties, because it's our time with all the kids. We don't make couples type plans when it's a family weekend. We could attend every invite we get and use the same excuses she does and bring the kids home early etc, but we don't, we respect 'her time', she should respect our time. H's ex is the type person where you give an inch, she takes a mile. My ex is the same way.
H's ex is a spoiled brat who lives with her mom and sadly takes full advantage of her. Mom though, has created this monster because she allows it. So, with that, I feel that there has to be a line drawn in all situations. There was really no agreement set in their divorce but that there is agreed upon, 'reasonable visitation' based on work schedules, activities, etc.
I think the biggest issue I have is when H communicates with me for more time...but says that it's because ex and her mom want this/that. Frankly, I don't much care for what they want/need. Their personal wants/needs aren't my concern, nor should it be my H's.
Whatever the case, H and I talked and he made it clear that despite what his ex and her mother want, he wants more time with the kids now that school is out and together we are arranging it. No more lies. *Hopefully*