Compatibility has layers
BPM and key are the quick checks. They tell you whether two recordings can plausibly share time and harmony without extreme processing. They do not tell you whether both songs demand the same space.
A sparse vocal can work over a busy groove. Two dense arrangements may clash even when every number looks perfect.
Phrases need to agree
Most popular music is organized in repeating groups of four, eight or sixteen bars. When a chorus starts halfway through the other track’s phrase, the blend can feel wrong even when the beats are aligned.
Mark structural changes in both songs: verse, pre-chorus, chorus, bridge and breakdown. Look for sections with compatible lengths or a deliberate point of interruption.
Contrast creates the story
Many memorable mashups join records from different eras or moods. The surprise works when the parts remain individually legible. Contrast is useful; confusion is not.
Ask what changes in meaning when the vocal enters the new setting. That answer is often more important than technical cleverness.