The only possible explanation I can think of is that the Mercians didn't expect the Saxon queen's forces to have additional support from the Danes, and so saw less of a risk in splitting the force. But still they CHOSE that battlefield (there wasn't somewhere with a bridge or water shallow enough to cross?), and dividing your army like that is like #1 on the battle tactics "Don't" list.
Also, why was the other Mercian so surprised that the Anglo-Dane force was only attacking one side? "I totally expected them to also split their forces, because surely they are as dumb as us!"
Also, why was the other Mercian so surprised that the Anglo-Dane force was only attacking one side? "I totally expected them to also split their forces, because surely they are as dumb as us!"