Should I start nutes this young?

1st time grower indoors using 125W of CFL for 2 autos
16 days old, 1 looks great, the other about a third smaller and yellowing. both have been in identical situations , (biobizz light mix soil, same pot size, same watering, same light)

Here is a pick of the ill one:

it was looking fine up until about 5 days ago when the yellowing appeared and it hasn't improved so I'm thinking it is a deficiency of Nitrogen or Magnesium? The yellowing stems from the center new growth and through the veins

Also, is this enough light for the plants? I know autos dont get so big so what kind of yields can i hope for?

Definitely weird considering they're using the same soil. I don't think it's an N deficiency or a Mg deficiency or it would be the bottom leaves affected, not the top ones. It looks like iron deficiency. Do you have a way to check the runoff water pH? It should be between 6.5 and 7