Help with setting up two mixers
*Also posted this in "The Music Studio" so mods feel free to delete or move as you see fit*

I have a Rane 57SL and a Pioneer DJM600 and I want to connect the two in my home studio.
Connected to the Rane are two turntables which I use both vinyl and Serato with.
Connected to the Pioneer are two Denon 3700s which I use USBs with (but would like the option of Serato as well if possible).
I have QSC K8 active speakers which are runing off the Rane Mixer at present.

I currently have the Rane connected via USB to my PC which I use for recording in Serato.

Now I have the Pioneer to add in. I know I can just plug the output of the Pioneer into a line channel of the Rane and it will all work. The problem is that I'll lose one channel from the Rane, rendering it useless as a standalone mixer. What I want is both sets of decks to send to the PC simultaneously so I can record 4 decks at once using Serato (control Cds or control vinyl) on one or both at the same time.

So the other way to do it is to run the Rane into Channel 3 or 4 of the Pioneer. This will give me effectively 3 spare channels on the Pioneer (to run 2 CD players) and full use of the 57SL.

My question is, how will this affect Serato playback and recording. If I keep the Rane USB plugged into the computer it will record just the output of the Rane right? If I then run the record out of the Pioneer into my PC I can record this signal but will I lose the ability to record in Serato?

Is there any way to route the signal through both devices and record in Serato? I'm guessing that unless I have a SL box with the Pioneer, I can't use that with Serato at all, or is there a way to use the rane without sacrificing a channel?