I'm sorry that your kitty (what's his name?) is having issues with constipation.
Ideally, you'd be able to figure out what's causing the constipation so that you can try to prevent it. But kitties do sometimes develop issues with constipation as they get older. How old is your kitty?
Prevention-wise, keeping well-hydrated is key. You're already doing great things by giving him wet food and putting out more water bowls. If he's not eating 100% wet, I'd try to switch him over. Is he usually a good water drinker? To further encourage him, put the water bowls in areas where he spends the most time, so that he never has to go very far to find one.
I'm guessing that it's the Miralax, not the tuna in oil, that's helping him. At any rate, tuna in oil isn't something that you'd want to give your kitty on a regular basis. But it's perfectly safe to give Miralax every day, and it is most effective as a preventive measure. My kitty was on it daily for 4 or 5 years. Because it's dose-to-effect, you can adjust the amount you give based on the results. Has your kitty's poo been hard or soft? If he's pooping, but it's hard, I wouldn't hesitate to up the amount of Miralax. But most important, don't take a break: give it every day. I dissolved the Miralax thoroughly in a little water before mixing it into wet food.
So right now, he did poo last night (Sat)? Then it's not an emergency (unless he can't pee). This was with 1 tsp Miralax twice a day? That's a pretty high amount, but if that's what it took to work, I wouldn't decrease the amount until you see that his poo is too soft. Then you can back off.
There's nothing like having a constipated kitty to show you just how much time you can spend worrying about kitty poo!