small jar, to bear a burden, load of two
儋 centers on carrying a physical load, especially a burden borne by a person. The specific sense of a small jar and a load of two are natural extensions of this idea of bearing weight.
儋 combines 亻 (person) with 詹, which likely contributes the sound and may relate to carrying or bearing. The exact historical development is uncertain.
A person 亻 beside 詹, which looks like a container with a lid and a handle. Picture a person carrying a small jar or a heavy load on their shoulder.
For タン, imagine a person carrying a heavy tan-colored jar: tan -> タン, and the person bears the burden.