Yes it looks like a firing order of 153624 would give you even distribution to all cyl. But you have to look at the valve timming not the firing order. If you plot the valve timming events you would notice that only the center port has equal time between events. In the front port when #2 intake is closing #1 is starting to open and the rear port is a mirror of this. It doesn't seem to matter until you try and make a lot of HP, and #1 or #6 goes lean
The guys in Brazil use FI or carbs with injectors as an auxiliary fuel systen on siamese ports. They have even said on dynos you can see the exhaust pipes are different colors because of the fuel difference.
I think what is happening is that when #2 valve is closing and #1 is starting to open the air stops until #1 can start to pull air in but the fuel falls out of supension.
In the old days guys with siamesed ports used different valve timming on different ports because of this problem.
I divided my head and have not had a problem since.
Harry
