first you can have only one identity column per table, second i didn't get why you need two identity columns in your table?? As even if you get success to have two identity columns, you will get same sequence as a combination of these two (for eg. (1,1),(2,2,)....) which is useless i guess.. so what would be the benefit of that??
see if composite key works well according to your requirements... or tell us more that what you want to achieve so that we would be able to help you out.
CREATE TABLE [Customer]
CustomerId int Identity(1,1) ,
CustomerNumber int Not Null ,
PRIMARY KEY (CustomerId, CustomerNumber)