It is a dilemma to bu sure. I have an 18' Lowe. I weigh 215 more or less (221 last doctor visit)
Another passenger about my size or lighter has very little effect on driving the boat with the big motor or fishing with the trolling motor. I have one buddy who is 6' 350 lbs more or less.
He can't fish with me. Standing on the back deck, the trolling motor steer all wonky. He can't stand for long. The last time he sat in the pedestal chair, I was worried for the chair.
2 big guys in a Jon boat? I'd be concerned in anything less than an 18 footer and even then I'd recommend a life vest that you KNOW will float you and that you KNOW won't slip off if you hit the water unexpectedly. Also you'll want some power to move you around. A 24 volt 80 lb thrust minimum. Any 12 volt trolling motor, even the largest one made, is going to be slow and not quickly responsive in turns. Just more stuff to think about.
Several years ago I was fishing in a relatively local stump filled conservation lake, came into a cove under trolling motor power, came across 3 guys high centered on a stump. They were all bigger than me. They were in a 16 or 14 foot Jon boat and they were STUCK. Jon boat has a 10 horse Evinrude. (In this lake, anything over 10 horse must be operated at wake only speed). I guess these guys wanted to go fast, I don't know. Anyway they had swung into the cove, under power and got high centered. Trying to spin off using the 10 horse, they struck a different stump, sheared the pin and the prop was currently at the bottom of the lake. They had a 12 volt Shakespeare trolling motor. Didn't know what was wrong with the battery, in their own words - "We charged it last week and it worked fine."
I had trouble keeping a straight face, but I knew if I was stuck like that I'd want someone to try to help me if they could, so I had to help.
Their first idea was to throw me a rope and have me pull them off the stump. OK. "We'll just hold on to the rope". Ok, I thought - bad idea, but ok. Needless to say the guy wisely turned loose of the rope before getting pulled out of the boat and was before the boat got pulled off the stump.
We should tie to the boat, I told them. Ok - they tied the rope to a mooring cleat. Another bad idea - the cleat broke. Tying to the front of the boat wasn't doable - couldn't reach it. Finally tying to the rings where you'd strap the back of the boat to the trailer worked - I got them off the stump. They were 3/4 mile from the launch ramp and couldn't make any headway against the wind with their tiny trolling motor, so I had to drag them - transom first, against the wind back to the launch cove. They got soaked. There just wasn't any place to tie to the front of the boat that anyone could reach. Oh, yeah, there WAS a boat cleat but it got broke pulling them off the stump.
Got them to the bank of the launch cove, right next to the ramp and said - See you later - I'm going fishing, good luck. In the chit chat before I left the cove, they just couldn't understand why the trolling motor would 't pull them. They genuinely believed that trolling motor batteries recharged themselves over time - just like car batteries do.
Anyway - rest of the story - never saw those guys again, never saw their truck in the parking lot, nothing. Maybe they learned something - maybe they didn't. Point of the story is that heavy guys in Jon boats need to be prepared for the unexpected. Don't count on me - I only fish that lake every other week or so - in decent weather, so I might not be around.