hmm… change the map settings in c++
auto Settings = const_cast<UGameMapsSettings*>(GetDefault<UGameMapsSettings>());
if (Vertical)
{
Settings->TwoPlayerSplitscreenLayout = ETwoPlayerSplitScreenType::Vertical;
}
else {
Settings->TwoPlayerSplitscreenLayout = ETwoPlayerSplitScreenType::Horizontal;
}