If condition in R based on multiple fileinput

In ui.R I have:

      fileInput('file1', 'Run code a',
            accept=c('text/csv','text/comma-separated-values,text/plain','.csv'))

in server.R I have:

    file1 = input$file1 
if (is.null(file1)) {
  return(NULL)
}

df = read.csv(file1$datapath)
... some code used by df

This is working fine, but I want to add another fileinput in ui, which then is used to fill the df variable.

so in ui.R it will look like :

      fileInput('file1', 'Run code a',
            accept=c('text/csv','text/comma-separated-values,text/plain','.csv')),


      fileInput('file2', 'Run code b',
            accept=c('text/csv','text/comma-separated-values,text/plain','.csv')),

but how do I make it work in server.R?

        file1 = input$file1 
if (is.null(file1)) {
  return(NULL)
}

        file2 = input$file2 
if (is.null(file2)) {
  return(NULL)
}

df = read.csv(file1$datapath)
df = read.csv(file2$datapath)
... some code used by df

So if I upload a file in file2 fileinput, then the df is filled with that csv.

This isn't working, but I think I have to use a if condition, but I'm not sure about that and don't know how I should do it.