Is there any mod that works that can merge two guildchats so the alt guild and main guild will get the same chat?

Well, just Create a channel called Gchat, set password or anything on to it, and use that as gchat.

Well, that is an option but it will take weeks before everyone learns to use the right channel so would be better with a way to merge the regular guild chat together.

You could try using GEM (Guild Event Manager) It's designed for putting up raid events ingame, but it also has it's own channel

There is no way to merge Guild Chat beyond using a custom channel.

Bliz only gives you one guild chat channel and no matter how hard you try, if you are not in the guild you can not gain access to the guild chat.

The guild manager can set everybody's access to not speak in guild chat. By doing that, it will force people to use the shared channel.