Q: How to edit the same iMovie project on 2 different MAC laptops

This is possible but not easy.  You need both to be working from a copy of the same library using exactly the same version of iMovie.  You can't work directly from a copy of the library 'in the cloud' but you could use the cloud to transfer updates of the library.  Libraries are often rather large so uploading will be slow.  If the library contains a lot of  other events and projects that you don't want to work on it would be best create a new library containing only the events and projects you need to collaborate on. 

Of course only one of you can be editing at any one time and you would have to notify each other when you have finished and uploaded an update of the library.  Its also essential to avoid updating the library while iMovie is running.

Another important point is to make sure that the library contains all the original media files and not links to files in other libraries like iTunes or Photos which won't work on the other computer.  To do this you need to 'consolidate media' For more information see:  http://help.apple.com/imovie/mac/10.1/#/mov882dee351

Geoff.

"I understand that we should only put stuff in there for the particular project that we are working on."

Yes this is to keep the library as small as possible.  

"am I supposed to move this library to the "cloud" and how do I do that?"

You need an account with Dropbox (https://www.dropbox.com/) or other cloud storage provider.  If your library is bigger than 5 GB (it probably is) you will have to pay for the extra storage you will need.  You both need a fast internet connection else it will take many hours to upload.

"He also needs to access clips and events from my laptop for a different movie project he's working on."

I would start with just one project and an event with just the clips needed for it.  This will be challenging enough.  If you need to send him clips for use in his project the easiest way would be to email them to him using Apple Mail's Maildrop with which you can send files up to 5 GB.  See:  Mail Drop limits - Apple Support

"And are you saying that after one of us edits the specific project that we need to save it to the "new library" so that when the other person logs on to iMovie the revisions/edits will be there?"

If each of you have your the library on your local Dropbox folder it will be updated automatically when either of you makes changes - but it will take time and you need to make absolutely sure that the library does not change while either of you have iMovie running.  In fact to avoid this risk (which could corrupt irretrievably the whole library) it would be safer for each of you to work from a copy of the Dropbox library that you manually update. 

Although all of this is possible I have never actually tried to do it myself and from your questions I'm honestly inclined to advise that it is too ambitious - there are too many ways that things could go wrong.  

Geoff.

From your original post " I need to "edit" an unfinished iMovie project that is currently on his laptop."  It is his library that you need to share to start with.

He has to make a library containing just what is needed for your project (making sure it has been consolidated).  He then has to copy it with the finder to a dropbox folder set up for sharing (https://www.dropbox.com/en/help/4459) and email you the link.

You don't have to do anything until you receive the link.  When you have downloaded the library  make a copy of it somewhere else, launch iMovie and File . Open library > Other and navigate to the copy of the library.  If this library works OK you will have got past the first hurdle.  By then you will also find out if the library is too large or takes too long to upload (could easily take 24 hours or more) for collaboration this way to be practical.

Geoff.

"login as "me / with my dropbox account on his computer""

I'm not sure how Dropbox will handle the fact that he now needs two different local dropbox folders - his and yours.   Will be interesting to see what happens.

"So then I should be able to see it in my dropbox folder right away."

It still has to be uploaded to the cloud and downloaded to your computer so I think the time needed will be the same.

Geoff.

With an external drive available to both computers and containing the iMovie library with consolidated media (see earlier post) it should be quite easy.  Just connect the drive to whichever machine you currently want to edit on.  Media imported directly from a camera or from ordinary files on either computer will be copied to the library but if you add media from a Photos, iTunes or similar library, the media won't be copied to the library and hence accessible on the other computer unless you consolidate the library afterwards.  Even if the media is available for example in both mac's Photos libraries I don't think the links to the media on one machine will transfer to the other.

If each computer has a different library that you want to share just make sure they have different names and copy both to the external drive.

Geoff.

I want to be sure of the situation:

So you have two libraries on the external drive OK? 

Can you still access the library from your mac on your husband's mac? 

Can you access the library from your husbands mac OK on your mac?

"Unfortunately now when we open the projects on his laptop the video clips are gone and there's a question mark saying

"Missing file" AND "Try reimporting the original source media..." in his iMovie."

Does this happen when your have the original library on his mac open and/or when you have his library on the external drive open?

If the clips on your husbands mac are from another application library on his mac and you did not consolidate before copying to the external drive this could explain the behaviour - but you would also get the same behaviour when you access his library on the external drive.

Geoff.