Overtired or undertired?

I am struggling with my sons sleep schedule (18 mo). We have been back and forth between one nap and two for a while now because neither is smooth. He usually needs a 5 hour wake window before bedtime and sleeps better at night if he has two naps. If he has one nap he seems to get overtired. But with two naps he usually fights the first nap or takes forever to get down. He will get fussy around the 3 hour wake window. He acts tired and wants to be held etc. but if I take him in to go for nap he fidgets, fights going to sleep (his eyes will be open and shut and almost asleep then will pop open), and takes 30 mins or more to go to sleep. I have a 4 yr old I can’t just leave indefinitely around the house by herself. But if I wait longer to put him down I’m dealing with fussing, inconsistent nap length and usually an overtired babe by the end of the day. I don’t know if he’s already overtired for that first nap or not tired enough. Help