import Data.Monoid

type TS a = [(Integer, Maybe a)]

combine :: TS a -> TS a -> TS a
combine ts [] = ts
combine [] ts = ts
combine ((i, xi) : as) ((j, xj) : bs) =
  case compare i j of
    EQ -> (i, getLast (Last xi <> Last xj)) : combine as bs
    LT -> (i, xi) : combine as ((j, xj) : bs)
    GT -> (j, xj) : combine ((i, xi) : as) bs

as = [(0, Just 1), (1, Nothing), (2, Just 2), (3, Just 3)]
bs = [(1, Just 2), (2, Nothing), (3, Just 2), (4, Just 5)]

test = combine as bs