Plumber - mounted routers

Question

I would like to create API with Plumber from more than one R scripts. I found documentation about mounted routers in Plumbers here. My main script looks like this:

library(plumber)

root <- plumber$new()

calculations <- plumber$new("C:/temp/R-scripts/Calculations.R")
root$mount("/calculations", calculations)

statistics <- plumber$new("C:/temp/R-scripts/Statistics.R")
root$mount("/statistics", statistics)

root$run()

If I try to call Sum function from the first file by this url: http://127.0.0.1:5787/calculations/sum?a=2&b=3 I get Internal server error and in RStudio I can see this message:

"simpleError in (function (a, b) { as.numeric(a) + as.numeric(b)})(a = "2", b = "3", a = "2", b = "3"): formal argument "a" matched by multiple actual arguments"

The first file looks like this:

#* @serializer unboxedJSON
#* @get /sum
addTwo <- function(a, b){
  as.numeric(a) + as.numeric(b)
}

Any idea what I am doing wrong?

Thanks

P.S. I forgot to mention that if I change the main script to:

library(plumber)
r <- plumb("C:/temp/R-scripts/Calculations.R")
r$run()

everything works as expected