In that case it’s really up to you which you prefer, personally I would use a separate drive for the projects, but if you have lot’s of space on the SSD that will work fine as well.
The limiting factor here is disk usage, if the SSD is busy reading/writing at the same time as your project files, it might slow the editor down a bit while it waits for access to read/write. However, placing it on a separate drive could potentially slow it down as well if the read/write speed is too slow. As long as your HDD drive is not really old, it should be fast enough.
Both ways have a limiting factor, but having it on a separate drive is my preference. The slowdowns I am talking about here are minor for the most part, but if the SSD is busy reading/writing a large block of date, the editor will likely freeze until it is able to use the drive and may cause slight stuttering.
Hope that helps!