You MIGHT be able to use the speakers from one HTS on the other if there was an advantage in doing that. I suspect there won't be and the speakers may not be compatible and damage the main unit. Use the one that sounds best to you or has the features you need.
If both systems are "Home Theater" - they have everything they need, speakers, DVD player, etc. There's no reason to connect them together, unless you need to put another set of speakers somewhere.
You MIGHT be able to use the speakers from one HTS on the other if there was an advantage in doing that. I suspect there won't be and the speakers may not be compatible and damage the main unit. Use the one that sounds best to you or has the features you need.