[iOS][Connect] Spotify won't "connect to a device" when the second phone's screen locked

[23449380]

Hi there, 

My phone is an iPhone

iOS 12.4.1

Spotify version 8.5.41

Connect to a device option won't show my second phone online unless I unlock its screen and open the spotify app. Once I do so, it will play music regardless if I lock the screen or not. If I just pause the music on my main device, I can not play it back because my second device will disappear again from "Connect to a device". Moreover I can not control volume neither. It won't up or down volume on the second device. Have tried while the phone is charged in or not. The same situation. 

Both phones are iPhone and the latest spotify there. 

Question 1. Is that Possible to keep my device always online ? 

Question 2. Is that possible to control volume from the second device ? 

Thank you, 

Royal A.