You can split them over 2 flights yes. It is generally 2 seats in CW per flight, sometimes there can be more but it's not predictable. The obvious thing to do is book 2 in CW and 2 in WT (or 2 in WTP if you are lucky) and then keep watching for further redemption options later on as an alternative to using 2 flights. Either way this will result in a change fee per paseenger but I guess the latter option ensures that everyone is on the correct aircraft if all else fails.