At six weeks (Vanessa) and five weeks (Siobhan) I observed that Vanessa is taller, but Siobhan is bigger, with larger fan leaves. This supports GIJose's comment that they are different strains. We'll see.
The attached photos are from Vanessa. Last week, there was only one leaf with these markings. Now there are three others. I'm guessing it's nutrient burn. I enhanced the lighting and the contrast to make the problem more visible. Any other opinions?