I don't know exactly what router you have, but I suspect it only has one transmission channel. Both ipads can connect at the highest possible speed the router can provide, but when both are being streamed to the router can only handle one transmission at a time, causing the network to slow down drastically.
Also, your network might be as secure as can be, but that will not stop it from being interfered by other routers that transmit in the same frequency channel. There are different programs/apps that can scan your wifi and give a good diagnose on whether there is much interference or not. You might get better speeds when switching to another wifi channel.