Audio Dropout when launching Serato DJ on DDJ SX2/SR2

Setup:
1. DDJ-SX2 powered up with Line-in music source on Channel1 with music playing. 
2. While music is playing, I connect my macbook to the DDJ-SX2 with USB cable.
3. Launch Serato DJ Pro (SDJP).

After/while SDJP launches, the music from the Line-in source temporarily stops for about 10 - 15 seconds.

Question: How can I connect and start using SDJP to the DDJ-SX2 with no disruption in audio while is is playing audio from a line-in source already?

Note, this is in a club and I am not able to setup the SDJP in advance of playing from the line-in source as the club starts playing from line-in early and where I get to the club much later.

I tried on DDJ SR2, the same problem happened.