One nap or Two?

We have been struggling with naps and crying/ screaming when he doesn’t get his way. Both, I think are signs of his age, but the naps have taken me by surprise. He’s not quiet 15 months, but he seems to only want to take one nap. It started around the time Petra and Mikey came up for a visit (which was so much fun, by the way). I would put him down after the usually time passed, he would go down great, but then sleep forever (3-4 hours). The first time, it freaked me out and I didn’t really know what to do. But I continued with setting him down for a second nap, which he would do fine, but play in his crib, never falling asleep even after the two hours. So I tried waking him up early from his first nap and giving a second, but he still wouldn’t fall asleep for the second. I tried this three times and cringed when I saw how tired he got before his usual bed time. Then, I tried giving him one nap, but after lunch, though he was clearly tired before, and the one nap didn't even last 2 hours. So I was at a lost for a while. Then, this week I gave in, so to speak. Yesterday, the day before, and today I put him down at a little bit later in the morning (around 10-11), and he would take one long (3-4 hour) nap. He seems to do fine as long as we stick to his usual bed time. He’s still sleeping 12 hours at night, so I hope that means he’s getting his full hours during the day and not getting too tried between sleeping times. Boy, being a mother is not easy –and there is a lot more trial and error than I expected.