Foil Freak 1211 said:
A 15k hitch is rated to tow 15,000 pounds total. If you had 18k towed weight, you would be over the hitch rating.
One other thing to consider before towing tandem is state regulations. Each state has different requirements, some do not allow towing tandem, most have an overall length limit. Trailer Life mag puts out an annual report with each states laws. It's a great resource.
Lots of good advice so far. Check the state laws. When I was in Delaware, it was a pain to pull my motorcycles behind the 5er. I could do it in DE, but there was nowhere to ride in DE, so needless to say I had to go ride out-of-state. MD had an overall length limit (55 ft), and VA said "Absolutely Not. " It wasn't a big deal in Oklahoma, where I bought our 5er. I took it to a welder two days after I bought it and had a hitch welded on, and towed a boat to the lake that weekend. As far as my truck was concerned, no problem. That was with my Y2K truck with an automatic transmission and no mods. Needless-to-say my next two trucks did it even easier!
18,000lbs with a 15K hitch... Yes, you are over the hitch rating of the 5er hitch.
Tongue weight. Your 5er is set up to have a certain balance. The weight needs to be distributed properly. A bumper pull trailer needs to have 10% - 15% of the weight on the tongue, and a 5er needs 20%-25%. Lets say you have, as stated above, a 13,000 lb 5er. You need 3000-3250 lbs on the hitch. When you put your boat on the back, the math gets skewed. As above, if you're putting a 5,000lb boat there, you will have about 500 lbs on the hitch on the back of your 5er. This effectively doubles the weight taken off the pin on your truck. Where you had 3250 on the truck, you now have 2750 (3250 minus 500), but now you have an additional 500 lbs on the back, thus messing up your distribution. This will cause handling problems. What you have actually done is effectively taken 1000 pounds off the truck hitch. You need to put AT LEAST 500 lbs of stuff up front. Or if you take 250lbs of stuff that is already in the back and put it up front, this will work, or 500 lbs of additional stuff that isn't already in the equation. And when I say "front" and "back," I mean in front of the axles of the 5er or behind them. Also, when you are moving the weight, the distance from the axles makes a big difference. If you have 100 lbs loaded 4 feet in front of the center of the two axles, and you move it 8 feet behind that center, you now have "400" lbs effective weight distribution. There is a formula for this, but I'm just wagging it because this is already getting complicated

I flew C-5's in the Air Force, so believe me, weight and balance was a BIG DEAL!
In short, you need the right amount of weight on the hitch to keep it from wandering on ya. And it's bad enough if you have one trailer out of balance, but now, if your 5er is wandering, and you have a trailer behind
that , it compunds your aggravation! Been there, done that!
Now, as to the original post, a small bass boat is going to be negligable. It will be hard to get your rig out of balance with a small boat, but just keep an eye on where you load your gear.
SOLER