Dual sim models? Or other solution?

[Hadesdude]

Hi,

I started a new job a few weeks now as a technician, where I'm travelling half of the time.
I got one Xperia T for private use and a crappy Samsung GT-S5610 for work (though autonomy is sublime).
Thing is: I want my own phone to be able to use Coyote, a GPS app and run music in the background simultaneously. Whenever I get called, be it privately or by my work, the sound should go down automatically.

The car (which I should get in a few weeks) has both AUX and bluetooth functionality, and so do my current cellphones (though photo quality is insufficient for work on the crappy phone).

So I was thinking of using dualSIM capability, but if I have to replace the Xperia T, I'd want it to exceed its performances, like using a Quad Core processor and 4G. For the music I also want extensible memory. (16GB internal memory only, is not sufficient!).
The French brand Wiko is getting close, but not close enough.

I've heard that running the three mentioned apps on the Xperia T might be a burdon anyway, with its Dualcore 1.5GHz and 1GB RAM.

Or is there another way to make calls from one phone supress the sound of another? Like connecting one to AUX and another to Bluetooth?

Thanks in advance!