They probably pair together fairly well, because the Fusion lacks the Bus Compressor. The Fusion has a high-frequency compressor, but that is different. The Bus comp really glues things together. The Fusion has a more full-featured EQ, so you may not need the EQ on the BUS+ as much, but it looks like it is post-compressor, so you might use it to add some final eq tweaks.
It’s hard to say whether or not it is worth having both, as everyone’s setup and budget are different. If you don’t need all of the features and different compressor modes of the BUS+, and just want a good bus comp, there are lots of options. The Audioscape Bus Comp, which is an SSL clone is nice, and the price is around $800. There are other companies that offer clones in that price range as well, such as Stam Audio, Warm Audio, etc. If you do a search you will find plenty.