Each will compound the other, if they had been on the same day then yes no as effective. Take a BA and Virgin destined to a.n. same destination, both outbound cancelled due to BAA industrial action on the 7th . Next day BA have 2 sets of pax to get on the next service, then Virgin strike and would more than likely try to get pax to destination using the other carrier (like when we took BA pax when they striked). Space for another carriers passengers will be very limited due to trying to get their own programme back on schedule.
Result... carnage !