Id like to know if its possible to have two areas (5, for instance) connected
to the same area 0 backbone, but not connected to each other (two isolated area 5)
It can cause some instability or problem to my system?
As long as each Area 5's IP addressing is unique it does not matter if the two Area 5's are connected or not, each Area 5 will inject it's own Type 3 LSA's into Area 0. If, however, you are doing summaristion on the borders make sure your summary addresses do not overlap, if they do then you will have conectivity problems.