Get a second SSD or a bigger one?

I want to have some additional hard drive space, even though I received help finding a way to reduce my data usage with render files.  I'm currently using the internal 1TB drive in the MBP M1 Max for my library storage and editing.  I have my actual footage stored on an external NVMe SSD (Samsung 980 Pro); my footage storage is my entire iCloud library -a local copy.  If I was going to move all my FCPX files to an external device to edit from, would I be better off with a larger external SSD, containing both my actual footage source files AND my editing drive with FCPX library storage (so all activity is on one drive and not crossing the bus in the MBP), or would it be best to have a second drive, and separate these activities?

Thanks for any advice you can offer.

Dear Mattj, I have space problems like you.
I have a M1 Max with 512gb of internal disks.
I tried to work with an external disk, a Samsung T7 of 1Tb.
I copied there my FCPX libraries. So my original clips were stored there and the render files too.
FCPX is very fast to edit my file (on HD resolution). I don't have a real "realtime edit" but is very near to it. Rendering is faster, very faster. But I have to say that I don't use a lot of effects or titles on my timeline. I use HD clip (Proress 422), adding some color correction, some neat video filtering, some title over the principal lane, some transactions, no more of this.

The problem was that when the T7 was full (my actual project is about honey moon so with a lot of clips), the reading operation on it began very slow.

So now, the same project, with the same library is on a Samsung 2,5" with 5Tb of space but no SSD, is a normal 5400rpm on a USB3 cable.

To be completely honest, I think that now my editing is a little more slower. Rendering is a little slower. But not so much looking the T7 performances. So... I'm not so sure that use an SSD on USB-C is so relevant as we think.
It depends of how much space you use on your project.

Thanks for the reply!  Interesting...
I have heard that editing on FCPX, has a speed limit that is below that of an SSD.  Don't know if this is true or not.  There are some other fast drive options, that aren't as fast as SSD speeds, that could offer a near same experience without the cost of the SSD's.  I have a 7200 RPM G-Raid on order for backups.  I could test it on there and see how editing is there.

I also just ordered another 2TB SSD to edit from (if just as fast as being on the internal SSD), or I can store "work in progress" library files there and edit on the internal drive.  My work flow is weird.  I'm piecing together footage from several years at a time, so I am building about 35 videos at the same time right now.

I tried turning off the background rendering, but I'm a rookie editor, so I keep going back over the same areas of the timeline and playback isn't good each time, so I like to keep the rendering on.  It's making for a lot of large libraries.