Connecting a second powered subwoofer

I presently have a powered sub connected from my preamp's left and right outputs to the sub's left and right input via RCAs. I am getting a second identical sub....can I simply keep the pre left connected to the sub left and then connect the pre right to to the sub's right input on the second sub? This would leave one input open on each this safe? Or do I need to insert RCA plugs on those open inputs? 
Hi Lula,
You have it correct. No shorting plugs required and yes it is perfectly safe.
Hang on to your hat:)