The Cyton has a common bus, called SRB2, that by default acts as the common reference lead for all channels. Using the GUI control panel or SDK serial port commands, you can disconnect individual channels from the SRB2 and SRB1 bus, and use those channels as + and - differential pairs. (Top and Bottom pins of each channel.)
So easiest would be to just do this with two channels, and have one channel for one person, the other channel for the other person.
All of the Cyton Ground / Bias pins are connected together on the board. So run one Bias to each person. Also seat the persons close together so electric fields and cable lengths are not too long.
If you want to have multiple channels per person, it may be possible to futz around with the SRB* settings to use SRB2 as a reference bus on one person, and SRB1 as a reference bus on the other. But this sounds tricky and may not work. Alternately you can just bus the + leads together as needed with your own cabling. Cyton pins IN1N, IN2N, INxN are the closest to the board and by default those channels have the INxP pins bussed to SRB2. More info on bussing can be read in the ADS1299 data sheet on the TI site.