Oceanic have their own rules, deviate 15 miles and descend (tracks are at least 60nm apart). If you are on a direct routing it makes bugger all difference, every country and airspace is different so only answer is check the relevant AIP. Most important thing as was said earlier is the squawk. There are filters on the radar system to stop flights showing on your screen that are outside your sector I.E. If you are in charge of FL125 to FL235, you aren't going to want to see aircraft above that, so you can filter them out. When you squawk 7700 the filters are over-writtem, so everybody sees you and can prepare for a sudden descent by moving any potential conflicts, so the squawk can be more important than the Mayday in busy airspace