The problem with putting the 2x4 flue pans side by side to make a 4x6 is the flue boxes would interfere and you would have too large of a gap between the pans and you would be using A LOT more wood, even if you turned the flue pans sideways, there would still be a large gap and use a lot more wood than your current 2x6. The best bet, like Perry said, to extend your current arch and maybe extend the firebox as well to make it a 2x10, it would use the same amount of wood as your 2x6 but increase your evaporation rate to about 50 gph or maybe more with forced air which I would recommend for an arch that long and narrow. Or you could sell your current flue pan and by the Leader Max flue pan, or the Sunrise evaporator flue pan, they both have flues that are double the depth of a normal flue pan giving you much more surface area; or you could consider a steamaway/piggyback, or a simple preheater hood if your just looking to increase evaporation rate without breaking the bank or buying a whole new unit. Best of luck.
2,000+ taps and growing
3'x12' Vortex, MAX flue, and steamaway
24'x48' timber frame sugarhouse