I use to use "Oxford-Locks" to protect my bike. They are like a spine and are really difficult to break open even with power tools. Bike theft in Oxford is really high due to the sheer amount of people in and around the city who cycle and I had a few bikes pinched before I started using "Oxford-Locks" and I never had another bike stolen after then. This was back in '94 through to 2001, but I am sure you can get the same type of locks. D-Locks are really easy to bust open and too often people put too much trust in just using one of them and nothing else.
With your two bikes, I would remove the front wheels of the two bikes, making sure you have the frames rear wheels meet in the middle, so the forks are at different ends. That way it will be really difficult for them to get the bikes apart as well as getting the wheels off etc. Most thieves will only go for things that are really easy to steal and even easier to sell on, so the noisier and more difficult it is for them to take the bikes, the less likely they will even bother going for them.