You may achieve this using itertool.chain()
with list comprehension expression as:
>>> a=[[1,2,3],[4,5,6]]
>>> b=[[5,8,9],[2,7,10]]
# v join the matched sub-lists
# v v your condition
>>> [i + list(chain(*[j[1:] for j in b if i[1]==j[0]])) for i in a]
[[1, 2, 3, 7, 10], [4, 5, 6, 8, 9]]
solved How to merge two list of list in python