Are you using Line Wrenches?

One "trick", sometimes it actually helps to tighten the fittings (which can sometimes break them loose, especially if the fittings are damaged or rounded off a bit), before attempting to loosen/remove the fitting.

Another possibility, you could undue the fuel-line at the fuel pump, and then use two box-end wrenches on the fittings to try to get them apart.

If that doesn't work, hack-saw through the fuel Line, then remove the carb or just the float bowl, then you can work on it at a work-bench with Boxed-End Wrenches, Also, that way you can remove the fuel from the equation and heat the fittings a bit if necessary. I have had to go that route before.

You can also try some PB Blaster. Or, you can soak the fuel bowl with brass fittings, overnight, submerged in penetrant.