Combining two meshes

Dear all

I want to have two regions and then combine them into 1 region (th=th1+th2) because some integrals are solved on th and some on th1. so I made the mesh as follow:

real xr=6., lsponge=60., Xmax1=100., yr=3., yside=50.;

border in3(tt=1.,0.){real t=tt^1.7; x=0.; y=(yside-yr)t+yr;label=2;}
border interior1(t=0.,1.){x=xr
t; y=yr;}
border interior2(t=1.,0.){x=xr; y=yrt;}
border axis2(tt=0.,1.){real t=tt^1.7; x=(lsponge-xr)t+xr; y=0.; label=5;}
border axis3(t=0.,1.){x=(Xmax1-lsponge)t+lsponge; y=0.; label=5;}
border out(t=0.,1.){x=Xmax1; y=yside
t; label=3;}
border side(t=1.,0.){x=Xmax1
t; y=yside; label=4;}
border interior3(t=1.,0.){x=xr
t; y=yr;}
border interior4(t=0.,1.){x=xr; y=yrt;}
border in2(t=1.,0.){x=0.; y=(yr-1.)t+1.; label=2;}
border in1(t=1.,0.){x=0.; y=t; label=1;}
border axis1(t=0.,1.){x=xr
t; y=0.; label=5;}
mesh th1=buildmesh(in3(60)+interior1(10
xr)+interior2(10yr)+axis2(80)+axis3(30)+out(20)+side(30));
mesh th2=buildmesh(in2(20)+in1(18)+interior3(10
xr)+interior4(10*yr)+axis1(50));
mesh th=th1+th2;

but when I solve the navier stokes equation and put the boundary conditions , it seems that interiors (1,2,3,4) cant be considered as interfaces between th1 and th2. in the picture you can see what happens. it seems that interiors cant connect two domains.

The error is due to the meshes sharing different lines (borders) as the contact. For the continuity, there should be one border. Use the common border while interfacing meshes. It worked for me.