You'll still be fighting the same mob, but you'll both encounter a similar level of difficulty as it's health etc is scaled according to your own level.
Essentially, things are still easier in a group as there's more people attacking the same mob. However, the loot/XP you each get will depend on your level. It's actually a good system as I can go to anywhere in the Broken Isles at 110 and still get the relevant loot etc from any mob as they will all be 110. It means there aren't any redundant areas.