system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] How would the Guild react to a Noble House doing their own space travel?

The Guilds' monopoly on interstellar travel stems from the monopoly on safe long-distance travel. Which stems from their use of prescient Navigators mutated through the spice.

The two key operating words here being safe and long.

What would happen if a Noble House had two planets within one Solar System under their fiefdom? In this situation, the use of pre-Holtzman drives that don't need a Guild Navigator is again feasible.

Pre-Holtzma FTL being so slow that their use outside of a solar system is logistically pointless but confined to one system their use becomes beneficial for administrative and economic purposes.
gpt
They wouldn't care. The important part about interstellar travel is the connection to the rest of the Imperium. You can call yourself a noble house all you want but what's the point if there's no one around to care? If you can't import vital supplies, including the spice that extends your life and that you're probably addicted to? If you have your two little habitable planets, good for you. No one in the imperium cares. 

If you try to cut yourself off from the imperium, well, the Emperor probably won't appreciate that and will send his Sardaukar after you. If the planets are worthless, maybe he'll just ignore you and you go back to being just some guy with nominal control over two planets until you die and someone else takes over and decides to rejoin the imperium - if they can get into contact with anyone to do so.