look into ent_buffers , ent_vertices.
ent_buffers wil give you access to the

triangles buffers (indices into vertices array ,3 vertices forms a triangle)
and the vertices buffers (an array of vectors-positions of vertices)

it will also return the amount of triangles in the triangle buffer
and ent_vertices if its a model will return you the amount of vertices

so you could also use ent_getmesh and ent_setmesh which deals with d3dmesh/buffers of your target meshes

if they are animated im not sure how that would be handled ,maby a parent and
child relationship (in model terms)

i guess theres more than one way to handle it(merging them)
depending on your situation but a start would be to look into those functions
theres also ent_status which would give you valuable information if you
need it..

if perhaps you could be more specific in details as to what you want
to achieve apart from merging meshes ,like are they animated or not etc etc