Well again there are 100 ways to do this.
What you need first is, that whatever scratch you will use to do it, you need good tempo controll on that scratch.
So the easiest is prolly to start with a basic baby scratch.
You take the sample part with the biggest oomph that you can scratch on from song 2 and baby scratch that, sometimes looping a low energy part of song 1 at 16 or 32 bars.
Then for example if the next song is faster I start to touch up the baby scratches a little faster and faster. If you are really good you adjust the pitch of song 1 a little upwards too. Then let loose song 2 and slam the crossfader on song 2.
This is the easiest way with only baby scratch. It can be much more cool with warp scratches, double cutoff scratch etc.
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