You can effectively mount two drives on the same mount point by using a volume manager. There are two main reasons that people usually want to do so: redundancy (each drive contains a copy of the data) or expansion (the space of the two drives is combined).
Assuming you are using a modern Linux distribution, you can use LVM for either purpose. You will need to create a volume group, then assign part or all of each disk to the volume group, and finally create a logical volume. You can then mount the logical volume on /.
The documentation for your distribution should contain more details on exact steps using the tools provided with the distribution.
There are ways to do it (Raid, jbod), what is your exact reason to do it?
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Oh gosh, the / filesystem was not installed as LVM. Any chance of converting it without formatting it?
I don't think it is possible without formatting drive. Also, if you combine two drives using LVM and in case one drive fails, you will not be able to access other drive too until you replace faulty drive.