Thank you all for the hints you gave. In the end this proved way to complicated for me so I dropped it...
Still I think having two internal interfaces would make sense, especially if one is a wlan device acting as an access point. Well, I might try it again if I can configure this from the web in LMCE 8.10.
Changing "eth0" to "eth0 eth1" in /etc/default/dhcp3-server , and adding a second 6-line subnet block in /etc/dhcp3/dhcpd.conf is too complicated? I wish someone would try it out and report back the test results, so I could find out whether my research into this solution actually works for that use case.
Serving a lot of simultaneous but different video streams to different rooms could need multiple inside LAN segments. Or avoiding QoS configs by dedicating a separate inside LAN segment to telephony (or home automation, or alarm video, etc). I'd like to see whether LMCE handles those configs by DHCP, which is LMCE's plug & play model, or whether something new needs to be added to the SW. Or just a GUI for adding the other segments and their respective configs.