I don't think you can stream output from other devices thru the nexus tv-out but I am not 100% sure. Regardless, why not just use your budget card for dedicated recording and nexus for live tv? That would eliminate the problem altogether.
I also just inquired about diseqc.conf supporting multiple dvb devices and diseqc switches. I'll be using multiple cards soon and each will have it's own switch and own diseqc requirements but at present it doesn't appear VDR diseqc.conf supports this. I haven't heard any comments back on it so that's all I can tell you at this time. The good news is that I don't think adding such support would be much of a problem.