Well, I am assuming they are not streaming from an Internet source, but a local file either on a USB drive or from a networked server. I would say the odds of an absolute perfect sync are pretty low, mostly because you'd have to enable the playback simultaneously for both players. Unless they are both next to each other and one person can press both remote buttons together, the chances of a slight difference between them is likely. The only way to guarantee a sync'd playback would be to feed both displays/TVs from a single Roku, and split the HDMI signal out to both sets.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.