You need to open the .th2 file in notepad++
and you will see that two lines that connected in the same point has the same coordinates in both.
for example:
line wall
162.05 -498.09
168.82 -472.44
181.19 -446.82
191.32 -423.1
197.27 -400.48
endline
line wall
197.27 -400.48
218.29 -367.04
234.37 -340.98
236.84 -312.57
243.02 -263.91
endline
all you need to do is add the second line coordinates to the first line and delete one of the duplicate points:
line wall
162.05 -498.09
168.82 -472.44
181.19 -446.82
191.32 -423.1
197.27 -400.48
218.29 -367.04
234.37 -340.98
236.84 -312.57
243.02 -263.91
endline
and you will get one line