So, the thing is, you’re on the right track. To use a Freesat recordable box with full recording functionality (like watching one channel while recording another), it typically needs either a second cable or a dSCR system.
The dSCR system (Digital Satellite Channel Router) allows multiple channels to be transmitted through a single cable, but not all setups support it, especially if your communal system isn’t configured for it.
If your current system doesn't support dSCR, then, yeah, the second cable option is your best bet. So, check with your building’s system provider to see if they can upgrade to dSCR, or if running another cable to your flat is possible. As for the Freesat box, yes, some of them support dSCR, but you’ll want to double-check the specific model you're interested in, as not all do.