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Just to confirm..
Each Pheonix lord belongs to an aspect. They have them given under the Aspect section of their profiles.
The generic Pheonix Lord section states they cannot join aspect warrior [b]sqauds[/a] that are not of their aspect.
So nothing prevents you running Jain Zar and Maugan Ra in the same unit of harlequins or wraithguard.
I'm not sure, I would guess that it is allowed, as I don't see anything that specifically says they can't. It could probably be argued that each Lord is in fact a member of his aspect, but don't see where it actually says that anywhere.
It says in each lords enry which aspect they are. They have an aspect field.. e.g. maugan ra's entry (the verbose one, not the army list) says: Aspect: Dark Reapers
However the Disciples rule only specifies you cant join aspect warrior 'squads' both ICs would join a non-aspect warrior squad, so I can't see an instant problem in the RaW.
The rule excludes them from joining other aspects not all units except the aspect they founded, if they wanted to do that it would make sense that it would read they can only join that specific unit. So rangers, guardians, warlocks, wraithguard, and harlies should all be possible. I have used Maugan Ra in harlie units before with no complaint from my opponent and have read about jain zar going with a small wraithguard unit in a transport in another army.
Aye, joining one or the other there's 100% no problem with. I was curious about joining two lords of different aspects to the same non-aspect squad. I can't see anything that prevents it since the preventative rule specifies 'squads of aspect warriors', so even tho the non-aspect squad may already contain a model of another aspect, it's not a squad of aspect warriors.