2, 3, 3, 9, 9, ?

3, 7, 3, 3, 7, 9, ?

determine the two missing numbers and find the rule,

Bonus. how far can each sequence go? Can you find a longer sequence of numbers that follow the same rule?

list
   10   for S=1 to 9
   20    gosub *Build(S)
   30   next
   40   end
  100   *Build(N)
  101   local J
  110   if nxtprm(N-1)=N then
  120    :print N;:AnyDone=1
  130    :for J=1 to 9 step 2
  140     :gosub *Build(N*10+J)
  150    :next
  160   :if AnyDone=1 then print:AnyDone=0
  170   return
OK

run
 2  23  233  2333  23333
 23339
 2339  23399  233993  2339933  23399339
 239  2393
 2399  23993  239933  2399333
 29  293  2939  29399  293999  2939999  29399999
 3  31  311  3119  31193
 313  3137  31379
 317
 37  373  3733  37337  373379  3733799  37337999
 37339  373393
 3739  37397
 379  3793
 3797
 5  53
 59  593  5939  59393  593933  5939333  59393339
 59399  593993
 599
 7  71  719  7193  71933  719333
 73  733  7331
 7333  73331
 739  7393  73939  739391  7393913  73939133
 739393  7393931
 7393933
 739397
 739399
 79  797
OK

In those cases where a line begins with a number longer than one digit, the preceding numbers in that sequence can be found on some preceding line, as the new line is a new branch off from a prior line.

There are five of length 8: 23399339, 29399999, 37337999, 59393339, 73939133.