You on a Mac or PC? ON Mac you could use the IAC bus to send MDII through the internal BUs. And you could sync them up with this internal clock. So when you press play in one, it starts the playback in the other as well.
The downside is you're running two DAWs at the same time and could be quite processor heavy depending on your setup.
There’s also a number of folk that run two copies of cubase / nuendo at the same time - using system link - which gains a few extra features over the otherwise excellent video slave software. Works a treat once you get your head around possible pref file issues. (I have used cubase as the video host and nuendo to write, but cubase only has one video track... and having a second for reconforms etc is a godsend!)