Adding different data in single textbox

I have two pieces of data, and one textbox. When one of the pieces of data is in the textbox, and I need to be able to add them both together and put them in the text box. Vice Versa for the other.

The data are:

Astart is a string (data)
Bstart is a string (data)
txtTabWrite is the text box

What is the code to do this?

Your saying that you want to append the txtTabWrite with results from Astart and Bstart?

VB Code:

txtTabWrite.Text = txtTabWrite.Text & Astart.Text & Bstart.Text

Yea but I need to be able to put Astart in the text box with just itself, and be able to put Bstart in the text box by itself. But when Astart is in the textbox I need to add the two in the command button, the same for Bstart.

I need to do that within two command buttons

Astart in AddA and Bstart in AddB