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How about using your phone to track journey while it is connected to a Bluetooth heart rate monitor. While it may be seen as a little odd to wear a HRM for a 30min walk (and potentially left on all day), IMO it's not half as odd as wearing two watches.
Also, I reckon once you've done this for a few days and seen that your HR pattern is the same each day, you would lose interest in monitoring HR so carefully (unless you have a funky HR!) Could use the HRM for other activities too.
I'd be more concerned about the obsessive journey tracking than looking odd wearing two watches!!