As far as I understand if they havent been done with their booster within a year then you have to start again.
If thats your case, then I believe the second jab has to be done within the 6th week and then yearly as normal...correct me if Im wrong though, Im only going from memory!
The way I understand it if starting a course ie not done within last year.
One Injection the one approx six weeks after then six months after then yearly (SP) But do check with your vet to be sure
Just had George done, he had the combined flu/tetanus and then had to have a booster 4-6wks later and then the vet said he will need another in 150-215 days later (roughly 6mths) and then annually thereafter.