Belgian residence for German blue card?

Hi
I'm a non-EU national, living and working in Germany on a blue card, and in a couple months time obtaining German residence permit (allowing unconditional unlimited work and stay in Germany only).
At the same time I have a Belgian boyfriend. We live together on weekends but during the work week I stay in my rental apartment in Germany since commute between two places is about 2h one way (couldn't do it every day).
In order to apply for Belgian residence I need to be legally living in Belgium for at least 5 years.
My question is, what should I do to be considered "legally" living in BE? Can I live legally in BE based on having my GE blue card (of course without the right to work)?
Or if I "legalize" my presence in BE based on my relationship with a BE national, can I officially be living in both countries?...
Sorry for the complex question, but perhaps someone has got an idea?... :)
Thanks for any tip.