Site Structure Question: Joomla=domain, Elgg=sub....something

I have a project that has taken a left turn. What was two sites, one Elgg community and one Joomla site has possible become one larger lump.

I'm much more familiar with Joomla then Elgg. Based on how Google treats subdomains as essentially separate sites and this project really is one larger site, I think it makes sense to use a subdirectory here. As I want all my SEO etc. to be tied together.

The Joomla side itself will not need any login (except for admin and a couple of editors) so I will not (hopefully) have to worry about a single larger sign on for both parts.

I was thinking about using Elggs Pages, but I really do need a larger website presence then that.

Anyone have pointers/thoughts/nightmares about doing the above?