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