Yes, SeedFAST agreements are not classified as debt which makes them SEIS and EIS compliant.
As long as you convert the SeedFAST investor to equity within 6 months of them sending over the funds, as per HMRC guidelines. To do so you must select a longstop date of no more than 6 months in your SeedFAST.
Important to note is that you would need to create two separate SeedFASTs for an investor intending to invest in both SEIS and EIS and the longstop date for the SEIS amount should be a day or two earlier than the longstop date for the EIS amount.
For more information please see our article Introducing SeedFAST, an SEIS/EIS-friendly way to raise ahead of a funding round.