I never got more than three good shaves from a cartridge. Five blades go blunt just as quickly as one - and of course they would, if you think about it.And if they lasted 30 days as advertised it would be a little less absurd, at least for the regular fusion carts, not the labs.
But 5 shaves and they are done for me. Unless I want to fill my neck with red bumps.
Nope…
I know the man who invented the twin blade razor at Wilkinson Sword. He was my neighbour at one time, after his retirement. He freely admits it was entirely a marketing initiative to sell twice as many razor blades (which is predicated on the blades wearing out at the same rate as a single blade). He doesn’t think it improved the shave in any way compared to one blade, but their job was to sell razor blades. It also locked customers in to buying that brand of blades, since other cartridges would not fit (and this is something the razor blade manufacturers have been trying for a hundred years, and the reason why the standard DE blade is such a ridiculous shape).