Probably not unless it's official. Meaning it's a risk and better to expect it not to work that way (even one might hope that it does/might).
I think I actually got sort of official confirmation for my iPhone
If I enter my AT&T iPhone's MEID on this page, Verizon recognizes the device: http://www.verizonwireless.com/b2c/nso/enterDeviceId.do?&zipRdr=y
Note: My iPhone 6 is a genius bar replacement and also has been unlocked by AT&T as of this morning. My dad's iPhone 6+ IMEI/MEID is not recognized on that page (but for whatever it's worth, his phone isn't unlocked)