I am running Kodi 14.2 and I have 3 IPTV clients. IPTV Simple Client, Stalker Client, and NextPVR Client. NextPVR runs fine under Windows with my WinTV 1600 card. Unfortunately, I have to disable Stalker Client in order to get NextPVR to get the channels to display in Kodi 14.2. Stalker and IPTV seem to coexist under Kodi. I am using Analog Cable.
My understanding is Kodi only really supports a single PVR client at a time. The 'IPTV' PVR plugin might be a special case because it's a very cut down plugin, and lot of people have tried to use this along side another PVR addon.
If you can post your kodi.log with debug logging enabled, I can tell you if kodi is at least calling the NextPVR plugin. My guess it isn't, and there is nothing I'd be able to do about it from the NextPVR addon end, and you'd need to talk to the kodi developers about getting multiple PVR addons properly supported in the future.
You can use multiple backends but you have to turn off use backend channel numbers.
Martin