You will need an HDMI distribution amp to split one HDMI into two but nless your soundbar can process lossless audio codecs like Dolby TrueHD there is no reason you can't connect it to the TV audio output rather than directly to the Shield. Optical digital and HDMI-ARC work fine for lossy surround codecs like Dolby Digital or DTS.
Even if the soundbar is compatible with the Dolby TrueHD is suspect you won't hear much difference anyway.
What you will be looking for is an HDMI 2.1 cable. Don't worry they aren't more expensive than other HDMI cords. This site has a good explanation about ARC (audio return channel) for HDMI https/www.digitaltrends.com/home-theater/hdmi-arc-explained-works-care/
Also here is a funny but worthwhile Linus video about an expensive HDMI cord. https/www.youtube.com/watch?v=MjJzibFTaqA