How can I pass a constant value for 1 binding in multi-binding?

If your value is simply a string, you can specify it as a constant in the Source property of a binding. If it is any other primitive data type, you need to define a static resource and reference this.

Define the sys namespace in the root of the XAML to point to System in mscorlib, and the following should work:

<TextBlock>
  <TextBlock.Resources>
    <sys:Int32 x:Key="fixedValue">123</sys:Int32>
  </TextBlock.Resources>
  <TextBlock.Text>
    <MultiBinding Converter="{StaticResource myConverter}">
      <Binding Path="myFirst.Value" />
      <Binding Source="{StaticResource fixedValue}" />
    </MultiBinding>
  </TextBlock.Text>
</TextBlock>