Well from that, I gather that they are limited in their effective ranges. Assuming you don't want to match them in galleons, a goldfish with a lumberjack would do nicely to pin down one of them, leaving the other open for attack at the flanks.
Also, since they are galleons, you could come in on the outside of one of the galleons, which would basically shield you from the other galleon's fire.
There is no sure fire way, and like I said, there are a lot of variable at play here. Disabling one outside of it's effective range, thus opening up the other to flanking is a solid tactic though. Terrain can play a big part too, as a galleon is the slowest turning boat. That depends on map. Further, I also assume here that the other boat on the enemy team is tied up with one of your allies. If not, it gets harder.
Positioning is everything to this game. Galleons are more fixed in their stance, so that lets you get a good angle before heading in to pin them down.