+1 for the multiple systems solution. This is how I've managed to do exactly the same requirement as yours (synchronous 8 channel HD playout), sorry for the bad quality of pictures:
Nine "cheaper" machines in a rack, 8 for playout and 9th to rule them all with custom CasparCG application to edit/save the playlist in tree fashion and send commands to all machines. Working for almost 3 years without maintenance and zero problems in 7 days/week usage, in 3rd year one machine needed some care. It's questionable if the single machine machine would have such long uptime and zero problems.