There are more than I want to bother with, but here are a few that come into play when designing a feed waveguide:
1. Long enough to allow a given f/D adjustment range.
2. Short enough to minimize losses.
3. Capable of mounting LNBs so they don't interfere with each other, the scalar or the mounting arms.
4. Room for a dielectric or 1/4 wave plate for circular feeds.
5. Access to set-screws to adjust skew.
None of these have anything to do with focal point optimization, but are probably as or more important to a successful product.
If Ku-LNB position can't be adjusted independently of C-LNB, then a corresponding cloud catching couldn't seems to be one of these factors?
Designing a feed is not like designing a bookshelf. There is science, art and magic involved, and a feed designer is faced with constraints, optimizations, compromises and complex interactions, many of which s/he cannot control. An example might be the topologies of focal clouds, which will vary with the frequency, dish model, manufacturing tolerances and surface condition. We want one-size-fits-all, but that often means less than optimal performance. Some feeds may work better on certain dishes, but you won't know until you try. Fortunately the differences are usually not important.
Modern feed design is best done by computer simulation rather than cut-and-try. This allows the balancing of conflicting factors, some of which are hard to measure. A few gross adjustments are possible with feeds, but building a feed with separately adjustable mouths for C and Ku sounds like a theoretical and mechanical nightmare to me. Adjusting one relative to the other is going to have complex positive and negative effects on both, many of which cannot be measured by typical FTA users. The more adjustments you provide to a lay person, the more likely they will screw it up to the point that nothing works.
My advice is to be happy and not to worry. A good feed will take the necessary factors into account and do a good job optimizing very complex demands. Your job is to adjust your dish to track the arc accurately, ensure the feed horn is in the right place, set the f/D and polarization skew correctly, choose LNBs that meet your performance requirements and have a switching/distribution system that causes minimal distortions and CNR losses to the signals. A lot of people here work hard to get all of these right, but there is so much room for errors that I would not be surprised if many FTA systems are not delivering their full potential. Any single factor I just described will have far more to do with enjoying good reception than feed design details.