Expanding the given expression, we get:

(tga+ctga)^2 - (tga-ctga)^2
= (tga)^2 + 2(tga)(ctga) + (ctga)^2 - [(tga)^2 - 2(tga)(ctga) + (ctga)^2]
= (tga)^2 + 2(tga)(ctga) + (ctga)^2 - (tga)^2 + 2(tga)(ctga) - (ctga)^2
= 4(tga)(ctga)

Therefore, the simplified expression is 4(tga)(ctga).