Check the plasma fluid levels.

Two songs = louder than one song

check if the polarity is reversed

Try going New York style and quickly snapping it over - also yell on the Mic and people won't notice.

This is one of the main reasons I use traktor. With 4+ playing decks I can quadruple my audio output level as opposed to only getting the exponential gains of two songs at the same time with serato.

But to address your issue.. When the volume spikes from switching decks, immediately load a new song in the opposite deck. But note it MUST be a different song and you have to be fairly quick about it.

Plasma fluid in your upfaders is low.

Regardless of what program you use... more signal equals more volume. Traktor does have a built in limiter that will cap the overall output but it can also smash the heck out of the signal depending on how hot you're running your individual internal gains.

Selena... the answer is to line up all the eqs and gains so they are the same, bring up the line fader for the channel you are swapping to, and quickly bring the X-Fader over.
This will avoid the double up of the audio thus avoid the spike in volume.