i think as long as one shoal is bigger than the other it would look ok(i would go for half as many neons as embers personaly)otherwise it may look too busy,ive got 20 embers and am going to be adding another 10-20 along with 8-10 congos,hopefully the congos will make the embers shoal fairly tightly due to the big size difference,at the moment the embers do shoal occasionaly but where they only share the tank with 4 sae and 4 ottos theres nothing to make them shoal and the look untidy/unnatural if that makes sense.
cheers dave