Inexperienced at etops, but do pond crossings on a regular basis.
Thought process would go like this
1) Diagnose actual snag. If the gauge is going down, is it a leak or gauge error? I would suggest noting the trim gauge whilst sending either a trolly dolly or poss First officer down the back to see if you were trailing. IF it was a leak, move on....
2) Pan call and turn towards nearest Diversion. Decend A/R 500' for oceanic par example
3) if fuel leak could cause fire, (IE SPRAYING near nacelle) SHUT IT DOWN!!!! you should have plenty of gas to get back with one full wing and centre tank. Might be wrong tho. Do some perf sums at worse case (PNR?) and see what figures you get.
2) Cancel Ctr tank feeding, and feed engines tank to engines. Would not advocate feeding both engines from a leaky tank... Instead operate at Max continuous, whilst also transferring fuel into Main tank and possibly centre tank from leaky one. as to Structural fuel limits, Its a tough call... Id try to remain in as long as in doing so, I was not sacrificing Fuel to get to diversion (assume tank empty for rough fuel calc)
3) Think ahead. Is it worth landing with fuel leaking all over the shop? Is foam laying available? Fire carts a must I reckon. I would personally shut the engine down before it flamed out, then just top up the other tank A/R. I wouldnt worry about fuel emmissions in the environment, Ive heard about 50t thrown out with no comeback from greenpeace! Personally, you practice assy all the time, and would much prefer to land with one shut down than land with both working and 2 KGs per second flowing out the wing.
As I say, not the most experienced, but thats what id do at a push. Why not ring your sim guys and run a scenario? Or speak to your head honcho pilot? The only stupid questions in this game are those that arent asked then come back to bite you in the ass.