Sorry, I don't quite understand your massive single sentence that contains over 100 words.

Hmm yes I've heard of others asking for the same thing about 2bsp attaching. However, it was noted that a lot of lag will come up since Halo has a hard time rendering the whole bsp, which is why bungie separated it into different parts for each level. It is possible, not plausible.

If anyone has time to help you, which I think is what you're asking for, they might, or they might not..

Okay, here's what i got, you ant to attach the two BSP's from a30 and make it a multi-player map, you'd do it yourself but you lack the skill and know how. But you also said that the game play would be so close you'd think you were playing campaign, which leads me to believe you're trying to make a multi-player map that also has objective\ai ect. good luck man.

He has been asking me to do this for him for over a year. Thing is, A long time ago I did a proof of concept map called Exodus which is exactly that. It's laggy, portals are messy and I had to edit the BSP to save some polies. The main issue is this is a mostly OUTDOORS map so very often you are actually rendering the whole A30a and b segments.

Here's the link:
http://hce.halomaps.org/index.cfm?fid=5891

Also, the RPG_Beta6.2 airbase is in the map... if anybody wants to mess with it and mod it feel free as there's nothing in the map itself but the BSP elements