Dual Subwoofer Help!!!

[Alpha Sixx]

OK I used to have 2 subs connected a few years back in my HT and they sounded beautifully but a friend hooked them up for me and I have no more contact with that person. So anyways I have a super cube reference and I really liked the low end on it so I bought another one and will pick it up this Tues or wed. So I did some experimenting and put a y adapter directly out of the back of my denons sub out to my SCREF and my older Yamaha sub with 2 separate sub cables and Y adapters to the subs also it sounded really good bass was alot smoother and filled the room better and I got both loud and deep bass loud from the Yamaha and deep from the SCREF but it was slightly less stronger I guess because the signal is now split, strength is not an issue I can turn the subs up or the change it easily in the GUI. But is that safe for my denon????? And I also connected the sub cable directly to my denon with out the y adapter but the y adapter on the subs low level RCA inputs and another cable out of its low level RCA outs to my Yamaha sub kind of like a daisy chain but the Yamaha's response wasn't as good as the 1st setup. Any help would be much appreciated. I am not an audiophile so please keep it simple guys. Thanks.