I would start with the 17-40 or the 16-35. Get your solo hammerhead, sealion shots in the can. Then once you have those put on the 15mm and you can go for the big school shots and if lucky up close whale shark shots.
Not two DSLRs, but this is a rig I just dove in Raja. Don't you hate it when the Dive Guide says "on this dive we can see mantas and pygmy seahorses".... and yes I did get photos of both, on one dive. Bottom camera is the Oly EM5 with the Oly 60mm Macro Lens. Top is the Sony RX100 with the Inon H100 and Dome.
This set up has the Oly with the Lumix 8mm Fish Eye. I did have to take the sun shade off the Sony RX100 or it showed in the Oly shots. In this line up I used the Sony more to shoot Video while taking photos with the Olympus OMD.
Thanks for the comparison, but remember no good deed goes unpunished, lol. I think many "were" excited about the Sony line but the lens issues have tempered that excitement. The OMD has caught on because it is a good camera and with it's line-up of lenses everyone can go out TODAY. Sold my D7000 gear, taking the Sony RX100 on the net trip and still on the fence with OMD vs. Sony Nex vs. Fuji. Question is can people (me) wait till spring for the new Zeiss lenses??? Will there be a new version of the Nex7 between now and then??? Interested to see more on the Nex6.