hiding the cart display page does not make any sense, they would not be able to see any items in the cart until item 1 where added....
any way, the error message is caused by using a different cart name. the code example i gave assumes the cart is named eCart1, your cart name is WA_eCart1. Slight difference, but it's enough to through off the asp processor.
you need to alter the code to use the correct cart name:
<% If WA_eCart_InCart(WA_eCart1, "1") Then %>
<add to cart button form for product 2>
<% End If %>