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
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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.