l1 <- list(a=1, b=2)
l2 <- list(b=3, c=4)

modifyList(l1, l2)
# > list(a=1, b=3, c=4)

modifyList(l2, l1)
# > list(a=1, b=2, c=4)

# Note: later arguments override elements of the previous arguemnts.