After many years of breeding these fish I had never the chance to have two clutches of eggs the same time. In other words; while the alpha man was guarding the eggs; I haven't observed that another sub-alpha was enforcing his chance to breed with another female simultaneous (in the same tank). Do you have the same experience or observed other behavior ?
For info : breeding tanks are 100x40x40 cm with plenty of caves and the occupation is always 4 males together with 4 females.
I have only had one spawn by a subdominant male in my zebra colony, and that was early on. I do know a couple people that have had two spawns going at once in their zebra groups, but I'm not sure how common it is.