Are the two buildings using the same transformer? If so what about something like Ethernet Over Power or some type of power line communication?
If they use the share the same electricity supply/source I would go for the Ethernet Over Power also, some nice and cheap product around. I would explore other options and would be slow to run cable across using trunking or any other support unless it is very very secure, away from general access so you don't risk it falling months/years down the line and damaging someone or thing... If I had to run the cable, run it alone.