Yes, it is possible to qualify for Supplemental Security Income (SSI) if you already receive SS disability income. However, the two programs may impact each other. Personal assets play a role in SSI eligibility, while SS disability income does not examine the financial resources of the recipient.
In general, if your SS disability payments exceed $861, you make too much to also receive SSI. If you can get both payments, your SSI will likely be reduced by your SS Disability payment.
Although it may seem like it's not worth pursuing both programs, in the right circumstances, you may end up receiving more. In addition, SSI allows automatic Medicare qualification without the 24-month waiting period imposed by SSDI.