State Management :: Access PreviousPage On Button Click Handler Of Target Page

I Have two pages source.aspx and target.aspx. Source.aspx has one button and other form controls the collect input. The button has its postbackurl property set to target.aspx. On target.aspx I have another button and other form controls that collect input. When the user clicks on the target.aspx button, I want the inputs from both source and target gathered to be inserted into the database. The problem is the Page.PreviousPage property is only available during the initial rendering of the target.aspx page. So when I click on target.aspx button, PreviousPage is now null.

One strategy of mine is to collect the information on page_load and store it in ViewState, but is there any other method that I should consider in this process. I think the cleanest approach would be to somehow access PreviousPage on the button_click event handler, but how can I access this property.