Sharing audio between android devices

[Android Central Question]

It seems like a no-brainier, but I couldn't wrap my head around it.
I have an old android phone (Moto). I connected it to a set of old speakers using the 3.5mm headphone jack. Now, I would like to play music using my other phone - either stream music from my phone to my old Moto over Bluetooth or WiFi, or somehow control the old Moto and make it play whatever I want (from Youtube, local library etc.)

How can I do that?
I thought the simplest way would be to use A2DP like my car stereo, but I couldn't figure how to make my old Moto an A2DP receiver.

Thanks!