Is it a stand alone speaker?
Is it possiple to connect another speaker(RDH-GTK37iP Sony Speaker) to my existing (RDH-GTK37iP Sony Speaker)?
Let's see. You could try a male 3.5mm split into two female 3.5mm. Then connect a 3.5mm aux to rca into each of those female ends and into each RCA port of your speakers.
Its just off the top of my head, and I bet there are easier ways to do this, and there's only two adapters/splits being used:
3.5mm male to 2 3.5mm female
and
2 of each: 3.5mm male to RCA males.
I used those two recommendations as examples on how they look like.
Since there is no OUT port on the speakers, there is no way to really connect them together on their own (like while on bluetooth), although that would've been pretty awesome.
Monoprice - 6inch 3.5mm Stereo Plug/Two 3.5mm Stereo Jack Cable
Steren - 3.5mm to RCA Audio Y-cable - Black