You're in luck, lefty yanagi's command about a 150% premium.
Whether you get a gyuto in place of both would have to be your decision.
A yanagi is very purpose built.....to cut raw fish. It does other things somewhat well too.....but it's built to excel at that one task.
If you don't make a LOT of sushi, and don't already have a nice sujihiki, I'd get that before I got a yanagi. A sujihiki cuts raw fish pretty good, but does other things very well too.
A santoku....I simply don't use. I'd much rather have a 210mm gyuto....and use my 240mm gyuto's even more than those. A santoku is just to dainty for me for real prep work. I do have a nakiri that I like to use though.
What other knives do you have?
In order of acquisition, I'd get:
Then start adding the fun knives like yanagi, nakiri, honesuki, etc.