On my 2006 ST42, we have a crossover line. When we bought the boat, the valves were open, but didn't seem to work.
We found a lot of black tar in the port Racor fuel filter so did a fuel polish. They used the pickup tubes as the intake for the fuel polishing system, and the lower crossover as the return to "stir up the bottom of the tank during polishing".
When they took the crossover hose off, they opened the valve to check and nothing came out of either. The hose too was clogged. A bit of compressed air into the fitting, and we got a flow of fuel. They ran the polisher 12 hours on each tank, then switched the intake and output (so they were drawing from the bottom) to get the rest of the crud as the pickup tube only goes down to about 2" from the bottom. Then ran the polisher another 12 hours. (They had two pumps and filter systems so we got it done in 24 hours.)
They had to change the filters three times during the first couple of hours in each direction. When they drew from the bottom of the tanks, the first filter clogged within about 15 minutes! All gunky and nasty.
An interesting note, the setup was obviously "self-engineered" (mounted to a plywood panel), there were two stages of filters, the first used a roll of paper towels as the filter media! The second was standard Racor filters. They started with 30 microns and each change went down. The "paper towel" filter was commercial and they said that it was common on over the road trucks. Who Knew!
One other note, I've been changing my Racors (30 microns) every 100 hours with my oil. I've been about 250 hours since the fuel polishing and was replacing a cracked injector line and decided to drop the secondary (on-engine) fuel filter and have a look. After only 250 hours, it was so clogged, that it had sucked in the sides and bent the metal caps! The other engines fuel filter was almost as bad. After replacement the boat responds much better and my idle RPM went from about 560 to 700.
I'll start changing those filters MUCH more frequently!