Hello guys!
Thanks for your comments!
Yes, the bodyshakers have to be powered with an additional amplifier. Every bodyshaker will need its own amplified signal, which means that for 2 bodyshakers, you will need a stereo amplifier. It is also possible to plug 2 bodyshakers on the 1 amplifier output but this will be more powerful.
So in fact I just put a Y at the output of my HC amplifier and I connect it to both inputs of my stereo amplifier...
And it works great!
Fred