If it's the same as a Garmin (edit: 810, 820) 920, 935 (which I think it is) you can connect to one phone at a time using bluetooth. You can connect to two or more phones but not at the same time.

I do it with my devices depending on if I have my phone or tablet with me. If I'm running on a treadmill I normally have my tablet with me, after my workout my watch uploads to the tablet. If I've been running outside, my watch uploads to my phone when I get home.