VA Disability and SSDI are two different programs with different standards and eligibility -
I never heard of an un-employable rating at the VA - thought it was always based on the amount of service-connected disability which they rate you.
As long as you are eligible, you can definitely get both benefits but unless death is imminent or you have one of the 200+ SSA Approved Compassionate Allowance Diseases then the process at SSA for SSDI has to be medically based on your condition now, if it can be improved and their medical investigation of your condition. You don't get a rating as such from SSA - either they rule yea or nay. This could also take awhile - the more detailed you are in your medical history, the more it helps speed the process. Get doctors to also concur. They do approve people with ALS and Kidney failure pretty fast too - with no after approval waiting period - for everybody else, there is a (5) month wait period for SSDI payments to start.
Sometimes SSA might refuse you and then you may decide to test their ruling in their court with a lawyer before the appropriate judge. Sometimes they might approve you but review your condition periodically if they expect your condition to improve.
You may want to read this page as well as the other link on the left of the page, it explains a lot:
SSA.gov - Disability Benefits | How You Qualify
It's Always Something . . . . Roseanna Roseannadanna