Assign color to 2 different geoms and get 2 different legends

If you use a filled plotting symbol, you can map one factor to fill and the other to colour, which then separates them into two scales and, therefore, legends.

ggplot(dat, aes(y = y, x = x)) +
  geom_point(aes(fill = a, size = z), pch = 21) + 
  geom_boxplot(fill = NA, size=.75, aes(color=b)) +
  scale_color_manual(values = c("orange", "purple")) +
  scale_fill_manual(values = c("#F8766D", "#00BFC4"))

ggnewscale makes this very easy:

library(ggplot2)
library(ggnewscale)

ggplot(dat, aes(y = y, x = x)) +
  geom_point(aes(color = a, size = z)) + 
  scale_color_brewer(palette = 'Dark2') +
  new_scale_color() +
  geom_boxplot(fill = NA, aes(color = b)) +
  scale_color_brewer(palette = 'Paired')

^{Created on 2020-01-08 by the reprex package (v0.3.0)}

Any scale_color can be used as usual. I chose scale_color_brewer for convenience