it's easy enough to hide the add to cart button unless Item ID 1 is in the cart, that code would look like:
<% If WA_eCart_InCart(eCart1, "1") Then %>
<add to cart button form for product 2>
<% End If %>
this code example assumes the cart is named eCart1.
the part Ray is saying is not supported and beyond what we can do in the forums is editing the cart display page so that if item 1 and item 2 are in the cart, you cannot remove item 1