You'll need to store the Id of the user that is currently selected before you do you databinding. One way would be to handle SelectedIndexChanged on your Project ddl so that you can grab the user id of the selected item in your User ddl then do the binding manually. Once the binding is done, then you can attempt to set the SelectedValue of the ddl to the User Id you had stored.
EDIT: Added an example:
In your aspx:
<asp:DropDownList ID="projectddl" runat="server" AutoPostBack="true" OnSelectedIndexChanged="projectddl_SelectedIndexChanged">
<asp:ListItem Text="Project 1" Value="1" />
<asp:ListItem Text="Project 2" Value="2" />
<asp:ListItem Text="Project 3" Value="3" />
</asp:DropDownList>
<asp:DropDownList ID="usersddl" runat="server">
</asp:DropDownList>
In your code-behind:
protected void projectddl_SelectedIndexChanged(object sender, EventArgs e)
{
string currentlySelectedUserId = usersddl.SelectedValue;
// Do your user databinding here based on project selected
usersddl.SelectedValue = currentlySelectedUserId;
}