Joining two ropes end to end, and still easy to untie would call for a sheet bend. Strong, reliable, super fast and easy.
If used for life support or some really critical application, you might consider a double sheet bend, or perhaps stopper knots on the tails. I've never used a double sheet bend; it might not untie as easily as the regular version.
Note: the sheet bend is basically the same as the bowline, except with two ends and no loop. It's also the knot of choice when the ropes are of a different size.