I’ve had the same issues quite a few times. Only way to do is little bit landscaping on both ends, build the tunnel and if i get the error, then try to build the doubling tunnel opposite way. So if you are building your track from say left to right, try the second tunnel from right to left. Sometimes helps, sometimes not. Also make sure that you are really building a double track, the one where there is no gap between the trackbeds. Sometimes the game builds a version with a gap in between the tracks, and that can not be built into the tunnel.