Which object repository we use in real time and can we merge 2 object repository and how?

There are usually two  types of object repository
1.Shared Mode Object Repository (.Tsr)
2.Per-Action Object Repository (.Mtr)

We usually use 1.Shared Mode (.Tsr)object repository.

We can merge two object repository. If you go to
QTP-->File-->Settings-->Recourses
You can use the Resources tab of the Test Settings dialog box to associate specific files with your test, such as  VBScript library files and Data Table files, and to specify  the object repository mode and file to use for your test

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