Your question title already suggests it. You need a two-way binding:
<spread:FreqSpreadUC FreqOne="{Binding TestFreqOne, Mode=TwoWay}" />
Alternatively you might register property metadata with your dependency property that makes it bind two-way by default:
public static readonly DependencyProperty FreqOneProperty =
DependencyProperty.Register(
"FreqOne", typeof(String), typeof(FreqSpreadUC)
new FrameworkPropertyMetadata(
null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));