It's not possible to encrypt 51GB of real-world data in 1GB space for the same reason that compression algorithms can't achieve 51x compression ratios. Given that, such a scheme presents some challenges if you want to maintain plausibility. Either,

1. Each filesystem lives within an allocated area and knows not to overwrite its neighbors' data.

2. Some filesystems (the real ones) are privileged and know their actual allocated area. Others (the decoys) think they own areas of the storage volume that contain hidden data and therefore have the potential to overwrite the hidden filesystems if they are written to.

In the case of (1), you need to be able to explain why your computer has unallocated areas filled with pseudorandom data. That is never going to pass the plausibility test, imo.

In the case of (2), a lot of effort needs to be put into making the decoys look normal while not letting them overwrite the hidden data. There are a number of strategies you could use here that would work, but it will never be as convenient or simple as dual-booting and the more convenient you try to make it, the less innocent a hard drive will appear under close inspection.