Here is the thing to take into account: a text box value is never null but can be empty.
So, for the two, a very short line to check would be
if (TextBox1.Text + TextBox2.Text != string.Empty)
However, it's possible that your requirement is not having non-empty text, but also not having the text consisting only of the blank space characters. If so, the check could be improved with a bit more complex on-liner:
if (TextBox1.Text.Trim() + TextBox2.Text.Trim() != string.Empty)
One note:
never ever use auto-generated names like
TextBox1
or
TextBox2
; they violate (good) Microsoft naming conventions and are not really intended for permanent use; you need to rename all such name to correct semantically sensible names using correct English spelling. It will greatly improve maintainability of your code.
—SA