@Coolzero82: For me, it only works with the CrazyCat TBS driver, not with the Hauppauge-driver.
@all: Just bumped my Raspberry to Kernel 4.19 (BRANCH=next rpi-update), copied the firmware from the Hauppauge Page (actually the soloHD firmware) to /lib/firmware, installed and configured TVheadend, and voilà, the stick works flawlessly with two channels received simultaneously! (Mind you: I did the watching on Windows - Raspberry, even 3B+, is not powerful enough to decode two HEVC-streams at once, it even struggles with one, not having a H.265 hardware codec)
I guess its some incompatibility between the firmware (Hauppauge even claims support since kernel 4.8! But only single tuner. Dual Tuner is only with 4.17 at least) and the 3.x kernel. The firmware in CrazyCat’s driver is the same as the one on the Hauppauge page btw.
I’ll just wait for whatever Kodi-distro comes out first with mainline kernel for Le Potato… Seems LibeELEC is working on it.