PMS detects the two tuners on my system running Arch Linux with a 4.20.8 kernel. However, still not support for the German channels 
I am afraid, you only have one option right now to further try to fix your issues: edit the Plex database 
First, make sure your system really detects both tuners, e.g with dmesg | grep -i dvb:
Here is my output, and you see that both adapter 0 and adapter 1 are registered.
[ 3.261607] em28xx 1-2:1.0: DVB interface 0 found: isoc
[ 4.568918] em28xx 1-2:1.0: Identified as Hauppauge WinTV-dualHD DVB (card=99)
[ 4.573080] tveeprom: TV standards PAL(B/G) NTSC(M) PAL(I) SECAM(L/L') PAL(D/D1/K) ATSC/DVB Digital (eeprom 0xfc)
[ 4.573092] em28xx 1-2:1.0: dvb set to isoc mode.
[ 5.872336] em28xx 1-2:1.0: Identified as Hauppauge WinTV-dualHD DVB (card=99)
[ 5.876511] tveeprom: TV standards PAL(B/G) NTSC(M) PAL(I) SECAM(L/L') PAL(D/D1/K) ATSC/DVB Digital (eeprom 0xfc)
[ 5.876518] em28xx 1-2:1.0: dvb ts2 set to isoc mode.
[ 6.103056] em28xx 1-2:1.0: Binding DVB extension
[ 6.118263] dvbdev: DVB: registering new adapter (1-2:1.0)
[ 6.118266] em28xx 1-2:1.0: DVB: registering adapter 0 frontend 0 (Silicon Labs Si2168)...
[ 6.121382] em28xx 1-2:1.0: DVB extension successfully initialized
[ 6.121385] em28xx 1-2:1.0: Binding DVB extension
[ 6.128634] dvbdev: DVB: registering new adapter (1-2:1.0)
[ 6.128637] em28xx 1-2:1.0: DVB: registering adapter 1 frontend 0 (Silicon Labs Si2168)...
[ 6.131257] em28xx 1-2:1.0: DVB extension successfully initialized
[ 6.131260] em28xx: Registered (Em28xx dvb Extension) extension
After that is confirmed, please remove your tuner from Plex DVR, stop PMS, backup the database (!!!) and remove the entry for your tuner from the media_provider_resources table. The below link provides more details. After that, start PMS and Plex DVR will hopefully discover a “new” device and now hopefully also identifies the two tuners correctly.
