Sure, here's the code

Code:
VECTOR normal1,normal2,normal_new;

CONTACT* c1=ent_getvertex(ent1,NULL,vertexID1);
vec_set(normal1,vector(c1.nx,c1.ny,c1.nz));

CONTACT* c2=ent_getvertex(ent2,NULL,vertexID2);
vec_set(normal2,vector(c2.nx,c2.ny,c2.nz));

vec_lerp(normal_new,normal1,normal2,0.5);
vec_normalize(normal_new,1);

CONTACT* c1=ent_getvertex(ent1,NULL,vertexID1);
c1.v.nx=normal_new.x;
c1.v.ny=normal_new.z;
c1.v.nz=normal_new.y;
ent_setvertex(ent1,c1,vertexID1);

CONTACT* c2=ent_getvertex(ent2,NULL,vertexID2);
c2.v.nx=normal_new.x;
c2.v.ny=normal_new.z;
c2.v.nz=normal_new.y;
ent_setvertex(ent2,c2,vertexID2);