2 Rental Contracts in Berlin. Is registration at both addresses required?

[Chikchik 0]

My friend signed a rental contract of a slightly over budget apartment 3 months ago, then he found an apartment within his budget and signed the contract for that one as well.
He is planning to leave the earlier contract after the minimum lease of 1 year (as stated in contract)
What should be his options?
1. Register at both apartments with one as secondary residence?
2. Register at one himself and sublet the other with anmeldung?

I have never faced this scenario or heard of it.