It's absolutely possible to do multiples. I've done a whole product pack of elf ears with this method.

Do you mean when you go to object mode?  You have to have created your width morph in edit mode for it to exist at all.

Either way, when in object mode, notice the "value" bar below the Shape Keys dialogue.  You can type a value in this or click and drag to increase value of a shape key in object mode.

I don't think you can create a shape key additive to another one (that is, with a previous one applied and visible in edit mode), but I'm willing to be corrected on that point.