Catalysis of the reaction: 2 L-glutamate + NADP(+) = 2-oxoglutarate + L-glutamine + H(+) + NADPH. This is a two-step reaction: (a) L-glutamate + NH3 = L-glutamine + H2O, (b) L-glutamate + NADP+ + H2O = NH3 + 2-oxoglutarate + NADPH + H+.