Why join them together? Once placed, they won't move if on relatively flat
ground. Also back fill about 1" all the way around the tie. One could
drill thru them in two or three places and drive rebar into the ground.
1/2 inch will do. But if you gotta join them, a half lap works good, still
need the rebar tho. A reciprocating saw or a chain saw. RR ties are 8x6"