Can I earn SSI and SSDI benefits at the same time?

Yes – particularly those who receive low payments from SSDI. Recall that the federal benefit rate for 2016 is $733 per month for individuals. If one already receives monthly SSDI benefits for $400 per month, then SSI could supplement that amount up to the $733 cap. To do this SSI could provide up to $333 per month in this example.
This is of course dependent on whether a claimant is even eligible for SSI benefits. Since SSI is needs-based and has strict limits on a claimant’s resources (an individual cannot possess over $2,000 in disposable assets), many claimants that are approved for SSDI may not be eligible for SSI.