Viet and Koichi are inseparable from each other.
I'm pretty sure it is an indivisible obligation on the contract.
As we all know, Tofu and Fugu are inseparable.
If something is not(不) divisible (可分), then it is indivisible.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Will also help to know how to read the word 可分.