Branching in a branch

[David Munt]

I am about to branch from an existing branch. The existing branch 
has tags set up to allow merges to and from the trunk and I have 
successfully merged changes from the trunk into the branch 
several times. The new (sub)branch will, ideally, allow merging to 
and from from the existing branch. I would like to know if anyone 
has attempted same and if so, what pitfalls, if any, were 
encountered. I anticipate that such a setup will require a number of 
tags to allow cross-merging from one branch to the other and vice-
versa. 

If anyone would like to share their experiences, I would be very 
willing to exchange ideas etc.

Regards,
David Munt.