I think TradeDangerous does not support loading up cargo and taking it further away than one hop?!
It does not support the notion of buying goods from two different stations to sell simultaneously at a third.
a) It's a fairly rare use case,
b) When it's a practical use case, there's usually a better option,
c) It specifically defeats one of the optimizations that allows TD to solve the "NP Hard" version of the BPC problem,
(The NP hard version is the one that figures out that the 4th-best trade from Sol which leads to a 3rd best trade at Barnard's star ... is better than the obvious trade from Sol which leads to a dead end at Alpha Centauri carrying biowaste for 1/cr a ton for a hop).
It's possible it could be added as an option in future, but for now, it's one of those things where if you find a practical use case where this is demonstrably the best thing to do, you can work around it, e.g. by reducing your supplied capacity to treat some of the cargo capacity as reserved.
I'm assuming here that you're not confusing "hop" and "jump"; TD will quite happily make as many jumps between systems as you like before docking and trading, this is controlled by --jumps.
-Oliver