We're a brother (27) and sister (25).
What time division would you recommend us?
a) Berlin -5.5 days, Prague 2.5 days
b) Berlin -4.5 days, Prague 3.5 days
c) Berlin -4.5 days, Dresden - 1 day, Prague 2.5 days
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or specific expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, see the FAQ for guidance.
It is very difficult to say because it also depends on your preferences. But generally I would recommend option c) since Dresden is really worth a visit! And it is directly on the way from Berlin to Prague, so you don't have to do any detour. On the contrary you can split a long journey into two parts.
Prague is very well connected to Dresden (take the DBB bus) and from Dresden to Berlin there are a lot of fast trains.