Matching the keys
There are many techniques to mix in harmony. Keep it safe and mix with the tonic key(both songs have exately the same key). You can mix from major to minor keys, do not mix baselines going from major to minor( or vice versa) . You can move up and down the scale using modulation mixing, another term is "energy boost" mixing, where you can go up one halvstep, a wholestep or more without overlaying the musical parts, use a drop mix or mix the tunes when one of the songs are in a drumbreak.
Tonic mix - mixing with songs with exactly the same key, ie C minor with C minor.
Perfect fourth - mixing with songs that are 5 halv-steps apart, ie. C minor with F minor
Perfect fifth - mixing with songs that are 7 halv-steps apart, ie. C minor with G minor
Chord shift - mixing with songs that are in the opposite chord, ie. C minor to C major, don't mix baselines on top of each other.
Modulation - mixing with songs that are not harmonic compatible but gives a "boost" or "lift" of the mix, ie C minor to D minor. Do not mix musical elements on top of each other.You can often hear this technique in "cheesy" pop tunes at the end of the song when the key changes to give the chorus an extra lift.
