Well, prosecution needs to have a legally convincing argument that indicates it is likely you have another encrypted partition you're not giving up keys to.
In fact, the situation is no different from this: say you're a murder suspect and a neighbour saw you carrying several large heavy sacks into your car and you drove away. Say what really happened is that you went and buried some bags of toxic waste in some location, and then went and buried a dead body in another location. When asked by prosecution, you confess to burying toxic waste and tell them where. The rest of the outcome of the trial depends entirely on whether you've successfully convinced them that you just buried the toxic waste.
If you are truly innocent, the prosecution might claim "oh they have extra keys that they haven't given up", and there is nothing you can do to prove them wrong.
Which is it? Does encryption allow you to hide from the law, or can innocent people just be proclaimed that they are hiding something and that they have to give up keys that don't exist?
It is one or the other, because encryption plus multiple keys makes you 'indistinguishable' from an innocent person who truly cannot give you a key that doesn't exist.
The tactic might work, but how well it would work would depend on what other evidence was presented that you do have another encrypted area. For example, if they analyse the partition you gave them the key to and show that it hasn't been booted in 18 months; they cross-reference the cached DHCP leases with the times you were known to have been online using that machine and find discrepancies; they might even have secretly imaged your disk a month earlier and show that a large amount of supposedly free space has changed content in the meantime.
(Maybe they even have you recorded telling someone that you have a second encrypted area on the machine.)
If there's no such evidence, then it ought to be pretty hard to convict you.
The point is that you look exactly the same as an innocent person.
You are taking the only possible pathway to being proved innocent.
It'd be like if I were to say "It doesn't matter what you do. The police are corrupt anyway, and will take you out back and shoot you no matter what. Guilty or innocent, if you get accused of a crime, you are dead."
And if they are going to lock you up no matter what, then you may as well use multiple plausible deniability keys. As you said, it doesn't matter what you do, the outcome stays the same.