Dividing Baltics between Lithuania and Estonia is not hard.
Having Daugava be the border is a lot more difficult, though. Historically, owning both sides of a river is a lot more beneficial then just one of the two, because cultures usually form on both sides of a river rather than just one (and also because you can then tax any shipping that goes through).
Same here - there were Baltic tribes on both sides of Daugava and there were Finnic tribes on both sides of Daugava. Plus, the Daugava Estuary (i.e. "Riga") is very important for trade, both to Lithuania and Estonia, and thus nether side would want to just split it in half.
Even if the border somehow starts at Daugava, there will be plenty of wars between Lithuania and Estonia over who gets the whole valley (and, imo, I would give Lithuania the higher chance to win, simply because of the population advantage)