It all depends on his size. If an adult mouse is just the right size for him (girth-wise) then I would recommend just feeding him one. If he's still acting hungry or the mouse didn't leave a slight lump (slight, not huge, but slight) in his belly, then you might try offering him another.
Do you have a scale? I feed my guy 10% of his body weight. Which right now is 3 adult mice per week. Also what's he feeding on? My guy was on fuzzy's when I first got him. I moved him up to two fuzzy's per week for a while then I would feed him one fuzzy first then an adult mouse. I'm not sure if this really helped him from moving from fuzzy's to mice but it worked for me. As he gets bigger I'll move him up to small rats and use the same process.
It should be fine as long as you're not feeding him too big of mice. I feed mine 2 every week, he takes them like it's no problem. He's only 5 months old, I have no scale so I'm going by the size of his midsection, I'll feed him 2 smaller mice..
I fed two for the first like month or two of owning my pastel. I did it cuz the mice were so damn short. Now that he is up to adult mice I dont double up. Maybe once at the end when I am about to upgrade his feeder size cuz I dont want him to like only expect one mouse then get up. (I dont want to condition him, dunno if if its even possible. But he did just fine, no signs of problems and is happy as can be at 330g+ after 4 month. Most I ever knowingly fed my snake was 17% of bodyweight. So its not like these were tremedously huge to be doubled up.
Go out and get a small kitchen scale to weigh your snake. It is a good idea to track their weight to make sure they are gaining weight and not loosing it. It is also helpful in gauging how much to feed them