You can do something very wordy
library(rmarkdown)
render("xyz.Rmd")
render("abc.Rmd")
This would execute the documents one after the other, in the same R environment.
If you want more programmatic way, you can follow @eringrand suggestion
library(rmarkdown)
rmd_files <- c("xyz.Rmd", "abc.Rmd")
# or if you want all the Rmd file in a directory
# rmd_files <- list.files(pattern = "[.]Rmd")
outputs_files <- purrr::map(rmd_files, render)
purrr is a package for iterations
If you don't want the same R environment to be shared, you can set the envir= argument in render() to new.env() or you can launch the file in a background process for a new fresh R session, using callr for example, or xfun::Rscript. It is more advanced, and you need that in some specific cases.
Hope it helps !