I am unable to get sounds from both my UE MEGABOOM speakers

I have synced my PC to my iPad. I used to be able to use the "double up" feature for both my UE Megaboom speakers. Now I get a message on my iPad saying: "You will only have one UE MEGABOOM available from your i Phone's AirPlay options while speakers are doubled up." And it asks me to verify by clicking "OK" Why can't I get sound from both speakers when controlling them from my iPad?