Let me explain our workflow. That would help see why this is important to us. We are a violin eLearning company. In one of our boards, each line item represents a client/musician. Since we like to keep the count on when we saw each student, we use the subitems. Each subitem is a separate date when we saw that student for a private course. It helps us know when the student was present, absent, late, and set each individual course as « paid » or not.
But we also have the curriculum with each student. So we have a saparate board with the same students, duplicated students info, and as sub-items, we have each particular music exercice we ask them to do, the scales, pieces, homework, etc. And to be frank, it’s a pain having to go to back and forth between the two boards. If we could just add a second subitem column, that would be perfect. Each student would then have the course occurences list, AND the program list. Easy to implement. If a subitem list is clicked, it would be as simple as closing the other subitem lists and opening the other, and vice versa. That would make us stay in one board and stop duplicating info about the students from one board to the next.
I can see a lot of uses where projects have different lists of subitems according to different departments for example. Each list would be separated as a different column, but linked to the same item. Anyway, that’s my feature request 