This is because a Smart LED with Multicolor Bluetooth Speaker bulb has two Bluetooth modules: the primary module is for audio streaming, and the secondary module is for lighting/volume control.
The primary module shows up as "Solo 2 RGBW” (circled in blue) in the Bluetooth settings page of your device.
The secondary module will show up as “S21XXXXX” (circled in red below) in the Bluetooth settings page of your device.
You would want to ensure the second phone is disconnected from the secondary Bluetooth module; the Pulse app is meant to utilize the secondary Bluetooth module while the main phone is connected to the primary Bluetooth module.
Normally whichever mobile device opens the Pulse app first will gain control of the secondary module, thus preventing any other mobile device from controlling lights via its Pulse app.
Ensure the second phone is disconnected from any of the Bluetooth broadcast by a Smart LED with Multicolor Bluetooth Speaker. This is best done by turning the Bluetooth off.
Then, on the main phone, connect to the primary Bluetooth module.
From here, open the Pulse app on the primary phone. This should allow your mobile device to gain controls over both audio and lighting as long as the phone is connected to the primary Bluetooth module—"Solo 2 RGBW”.