how to union 2 repos

Hi all

quote from core-tutorial: ######### begin quote Note You could do without using any branches at all, by keeping as many local repositories as you would like to have branches, and merging between them with git pull, just like you merge between branches. The advantage of this approach is that it lets you keep a set of files for each branch checked out and you may find it easier to switch back and forth if you juggle multiple lines of development simultaneously. Of course, you will pay the price of more disk usage to hold multiple working trees, but disk space is cheap these days. ########## end quote

I still can't realize how to union 2 git repositories with linear history, into 1 repo with 2 branches. i try git-rebase, but results are unexpected.

i have 2 repos with history, assuming initial commit in both repos are same:

1: a-b-c-d-e-f 2: a-B-C-D-E-F

i want repository with this history:

a-b-c-d-e-f
 \B-C-D-E-F

i try following:
1. copy objects from other repo :
git-fetch ../other/.git +master:otherMaster
history after this command is 2 unconnected chains:
a-b-c-d-e-f  a-B-C-D-E-F
2.Then i try following:
git-rebase --onto a f F

but results are strange (git, complain about merge errors, history is like:

x-x-x-x-x-x-----------x
 \x-x-x    \x-x-x-x-x

)

can someone help me how to do this ?

Thanks in advance