You have obviously been very sensible with your pen making and put in a lot of effort.
I find that the single blank pens sell better too and the good thing is they are easier to make. Because of the extra effort to make the double pens I put a little bit more on the price and that is what seems to turn people off.
Is your mandrel just the type with the flat milled on it and the small roller to lock the blank onto it?
A WIP of a closed end pen would be great.