I start the rear first on the O-2 because my battery is just inside the baggage door, plus, you can clear the rear engine verbally without the front one running. Yell out "Clear Rear", verify no one present and crank. I can visually clear the front prop.

With the front engine running, you cannot verbally clear the rear over the noise.

this is interesting. My POH doesn't specify which engine to start first, but makes the point of the shorter cable run to the front engine, while also pointing out that if you start the rear engine first, you can hear it and let off the starter sooner. If my battery was so low as to barely start the plane, and I was launching into IFR conditions, I might choose to charge the battery on a battery charger first just to be sure the battery was well enough charged to keep the battery contactor open when a heavy load was placed on the electrical system, like raising the gear. Just a thought. There is a supplement in my POH about vacuum pumps, saying to check the vacuum manifold on start up and shut down on each engine, and to shut down the engine you started first, first;to be sure the other vacuum pump is working, and the manifold is working as well.