that's partly true, but i think it's better to have unsightly join then only be able to see your fish from top view all the time as in the case of indoor pond.
This has been done and it works... your glass thickness does not play a part in this as the thickness was already considered for the depth of the tank... the only issue which needs to be considered is the bracing which needs to be added in the center.
Personally on that scale I would be considering some sort of full frame for peace of mind... but I'm un naturally coutious
Some sort of Angle Iron frame around the top and Bottom to help hold the two together and keeping the bowing to a min adding in some extra top cross braces...Could be a SICK TANK! although personally I don't have a use for a 12 X 2 foot print so you won't find one in my house anytime soon
If done properly it should have a clean professional appearance and be just as strong. Size doesn't matter. Johnptc basically joined two 5,000 gallon tanks to make one 10,000 tank.
I think it could be done well if instead of completely removing the adjoining end pieces, you cut out the centres, leaving say 25-50mm (1-2") around the edge, then silicone the two "donut" faces together.
This is something ive been thinking of recently after having a few qoutes for an 11 x 2 x 27".
At the moment ive got a 6x2x2 right next to a 4.5x2x2 and was thinking couldnt you just cut a hole in the end and have a tube of glass to join them both up with enough bracing etc or like oosh said,cut a u shape out of the ends and brace the top and glue the ends together?