Combining/Merging FCPX Libraries

So I came across a problem, and am interested in learning more about what the best way to fix it is. I believe it involved merging FCPX Libraries, but I want to be clear, before I mess up anything else.

Problem: I had two libraries on separate drives that were both from the same overall client/project. "Library A" was 1.05 TB and contained some projects and footage. "Library B" was 1.35 TB and contained all of "Library A" + more footage/more projects and more complete versions of the projects from "Library A". I needed to pull up this footage for my client and make a promo, but mistakingly started working in "Library A" and put several hours of work into creating a promo in that library. In creating this promo I imported new assets and music. It wasn't until i started looking for specific footage that I realized I was in the wrong Library. I really don't want to have to re-import/build my new Project all over again back in Library B, so I am looking for a way to merge/combine the Libraries.

What is the cleanest/easiest way to move my new project/newly imported assets from Lib A to Lib B?

Oh, and here is a challenge, both Libraries are on two separate 2TB drives, and I dont own any drives larger than 2 TB.

I hope this all makes sense.

Off the top of my head, import the new media into Library B. In Library A, export the Projects as FCPXML files, import them into Library B as FCPXML files. Then once Library B is all up to date and such, kill off Library A and avoid future confusion.

Update:

So I wasn't sure how I felt about exporting XML, so I actually took stab at something before your method and it worked! I plugged in both hard drives (one with Library A and one with Library B ). With both Libraries open in FCPX, I simply dragged over the newly imported media from A into the more complete library of B. I also clicked and dragged the Project File to B as well, all from within FCPX. Once moved over, I ejected and unplugged the drive with Library A, and B now included all of my new items and projects and played without issue.

I have since deleted Library A as recommended and will refrain from making Library versions in the future.