Not currently an officer in a raid guild, however, happy to comment.
Do you require an attendance level from your raiders? If so problem solves itself.
Depends on what you call a raid guild. My definition of a raid guild would be making consistant progress in content, attempting to kill mobs in the CURRENT expansion and possibly even getting server first kills / server discoveries. Members performance is paid attention to and if they are not pulling their weight they will get bumped for soeone who does. Membership in the guild does not mean a spot on the raid.
Some guild like to raid, but are not what I would call a raid guild. They get 24 people together to hit KoS zones and a mob or two in EoF zones. Allow members to float in and out for the raids, and if are missing a few key classes the 6 or so regulars rotate in their alts to cover those. problem here ends up being the raid force never seems to really get geared up. Progression stalls and then you bleed players, and the cycle starts all over.
Some raid guild only raid 3 nights a week and don't place any other time requirements on their members. Would think an alt in another guild would not be a problem as long as your raider in your guild was showing up to your raids.
Our guild raids 5 days a week.

Unless the person has nothing else to do and joins a guild in a completely different time zone I do not see how they could pull it off.
Why raid with an alt and not your main.. makes no sense to me.
So if I were running a guild and trying to really do some serious raiding, I could see allowing alts in other guild if and only if there were some set in stone attendance rules for raiders in my guild getting spots on the raids and getting loot. If that persons alt in another guild starts impacting my raids that person needs to make a choice.