Suggested Approach to have 2 D7 stacks -> 1 Civi Stack during upgrade/migration

[sonicthoughts]

I've read through the wiki notes http://wiki.civicrm.org/confluence/display/CRMDOC/Multi+Site+Installation and http://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=86213708 .

I would like to confirm what is necessary for a basic scenario:
Rebuilding/migrating a new D7 site with the same civicrm backend for beta testing.

OldD7->civi
NewD7>civi

I understand that civi needs to have a unique CIVICRM_TEMPLATE_COMPILEDIR and CIVICRM_UF_BASEURL. 

The latter doc above states:

Level: Separate domain records but the database is not configured as a mutlisite
Impact: This basically means you specify the domain in your civicrm.settings.php file. CiviCRM is not really aware of this. Where a table is domain specific it will select the rows that relate to that domain.

The doc then states:

Various settings, urls can be domain specific in CiviCRM Core - Settings table. This can be configured by domain or overridden at runtime in the civicrm.settings.php file - the latter is a good approach when only settings are dependent on the domain being accessed as it is simpler

Great, so I would expect that I just need to indicate the $SERVER domain in my civicrm.settings.php and adjust the variables above ... but the docs also suggest using the multiuser extension:

This permissioning is delivered by the the multisite extension. The extension also helps manage a shared user table (even if you turn permissioning off by setting domain group to 0)

I tried to enable the extension and it didn't work (white screen, no error message - does not show enabled.) I'm just not sure if it is necessary and I should spend the time to debug.  I can clone the civi database and work on a separate instance but would prefer to start preparing on a single server.  I'd appreciate some insights on the best way to approach during the brief migration window.

[petednz]

(possible off-topic comment) bear in mind that you need to have a plan for your uf_match tables to avoid problems later on eg if you create users on your d7 test site