Yes, you aren't limited to getting one per 28 days.Apologies if this question has been answered up-thread, but I didn't see it and the FAQ on Nation Rail Enquiries does not address it either.
Is the 28-day period set in stone? i.e. I plan to use my ticket 2 days a week, but stuff happens at work resulting in the 8 days being burned up in, say, 3 weeks. Can I purchase another Flexi straightaway even though the 28 day period of the previous ticket has yet to expire?
Conversely, if you end up not using your 8 tickets within the 28 days (or indeed even before the 28 days are up) you can get a refund. You'll be refunded the difference between the cost of the "Flexi Season" and the relevant number of Anytime (Day) Returns, less a £10 admin fee.