Actually it's easy then: you just move around them with flank mode with one ship while the other is firing and moving back and when the first one gets behind them they should already be with their backs turned and belly-side up so that you can do some serious damage. You can also hide behind debrief or try and make them collide/shoot (into) each other.
The hard part is when you have to face 5 destroyers that are as fast as you, that's when things get interesting and you kind of start using obscenities :)
Also get the latest patch (v.1.2) if you haven't already, it fixes some crashes and adds a "Draw Planets" video option. You can find the patch and the release notes
here.
It's great that Brendon will keep on working on the game and fixing bugs and listening to feedback from the community.