Consolidating 2 Tables with same Values
Sometimes, the lack of an uint type in SQL Server can come in handy. If Order-id is defined as identity(1,1) in DB2, you likely do not have any negative Order-ids. 

Consider importing all the legacy DB1..Orders.Order-id fields as the same number, but multiplied by -1. If there's a FK to Order-id in DB1, you would use the same multiple-by-negative-one conversion when those records are loaded into their corresponding tables in DB2. 

With this method, you also have the benefit of easily identifying the legacy orders originally from DB1 because their Order-id is always negative (just be sure to document this.)

Edit - added an example

-- Assumptions:
-- DB1 and DB2 are on the same server
-- The table Orders is in the dbo schema
-- Order-id is an integer-type in DB1 and DB2
-- Order-id &gt;= 0 in DB1 and DB2
-- Order-id is an identity(a, b), where a and b &gt;=0 in DB2

-- Overview: Insert records from DB1 into DB2, but reverse the
-- sign of Order-id (i.e., positive to negative).

Use DB2
Go

Set Identity_Insert dbo.Orders On;

Insert dbo.Orders
(Order-id, col2, col3)

Select (-1 * Order-id), col2, col3
From DB1.dbo.Orders;

Set Identity_Insert dbo.Orders Off;
Go

-- Have related records? Insert them here.