Baltazar,
If I write out the individual notes of each scale it would look like this:
A Major Scale: A B C# D E F# G# A
B Major Scale: B C# D# E F# G# A# B
Notice the notes that each scale have in common: B C# E F# G#
The notes not in common: D/D# A/A#
You would think because both keys have 5 notes in common they would mix well together. This is most likely not the case because of the Chord progression and Harmonies under the main melody. Again use your ears. Like GJ Fukit said, “if it sounds good, go with it, if not, forget it.”
If you are interested in learning how to mix songs in key I highly recommend the course “How To Master Key Mixing” when it becomes available on Digital DJ tips. They have a section called “A Complete Theory & Practice of Keymixing For Digital DJs” that will answer many of your questions.