Using 2 smartphones 1 for phone calls and the other for music

I searched the forums, couldn't find this topic.

I have a 2014 TL SH-AWD.

I have iPhone 5 for as a work phone and a HTC One M8 as a Personal phone. Both the phones are paired.

My ideal scenario is use the iPhone for phone calls using bluetooth and use my HTC for music streaming via bluetooth.

Sometimes automatically I see my both phones are paired iphone for phone usage and HTC for music usage.

I'm not able to replicate this at will.

What is the sequence I should follow so that I do not have 2 switch between connecting one of them at a time?

Thanks,