Two Database - How To Change Indexing Of Primary Key To Skip Some Numbers

I have two databases called

a) Old Retailer database
b) New Retailer Database

We cant delete old database but we are using and we want to use both of them.

Now problem

Both dbs have a table called retailer.

I want to prevent duplication in them.

I.e it there is a retailer 12 created in old one new one shldnt allowo us to create retailer

id---12

similarly if there is a retailer 13 in new one it shouldnt be created in old table

retailer ids are unique. its not the case that with retailer id for example 5 there is a

different retailer in these two tables of diffferent DBS. Its unique

Now I have to put this criteria

how can i do it?

Second questions===================

if a primary key is auto increment. how can i skip some numbers lets say it is 1,2,3,4

i want that when next record shld be inserted it shld be 6 not 5. i dont want to feed the value 6. i want to know how to change indexing of primary key to skip some numbers when its set to auto increments.