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

We can merge two object repository.If you go to 
QTP-->Test-->Settings-->Resourses
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

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

we cant merge two O.R

As far as i am concern u cant merge two O.R in a single
project i am speaking about shared object Rep(.tsr)

when speaking about per-action(.msr) this is possible
because for every action u perform a separate O.R is created
but u cant merge two files 

any queries abhesheke@yahoo.com

as said there r 2 types of object repository i.e.
1) per action 
2) shared mode
by default its per action but in real time we use shared mode.

yaa we can merge two obj rep by an option " merge object
repositories in Quick Test Plus.

hi guys..

   we can merge two repositories..but it is suggestible.

If u need two or more repositories for a test..u can
associate them by using associate repositories( If u r using
9.0 or above version) tool. 

If u want add it during run time..

rpositoriescollection.add "<repository file path>"

if any queries..
nanda.dreddy@gmail.com

We can Merge two object repository into one O.R. By using
the merge tool in the QTP 9.2.. 

Resources--> Object Repository Manager --> in that Tools
Menue--> Object Repository Merge Tool--> we can add primary
file and the secondary file---> select the path of the
repository files and click ok.

Now both the repository got merged and we can save this file
with the extension.tsr. and save the OR..

This How we will Merge two OR.

Note: But at a time we can merger two files only, not more
than that..

if you have any doubts shoot a mail: sudeepth.z@gmail.com

Merge is possible in QTP 9.0 version also in the method 
that is mentioned in the above answer