Multiple Selection From Dropdown Using if-else Rshiny

Question

I am trying to create two dropdowns based on user input. If a user selects "val1", then show values in the second dropdown from "x". If "val2", then "y". If both "val1" and "val2", then z.

So far, I succeeded except for the multiple selection part. I also tried:

else if (input$lab == "val1" & input$lab == "val2")

which gives me the same error:

the condition has length > 1 and only the first element will be used

x <- c("x1", "x2", "x3")
y <- c("y1", "y2", "y3")
z <- c("x1", "x2", "x3", "y1", "y2", "y3")

library(shiny)
library(shinydashboard)
library(shinyWidgets)

ui <- dashboardPage(
  dashboardHeader(title = "title"),
  
  dashboardSidebar(
    sidebarMenu(
      menuItem("xxx", tabName = "tab1")
    )
  ),
  
  dashboardBody(
    tabItems(
      tabItem(
        tabName = "tab1",
        pickerInput("dropdown1", "Values:", choices = c("x", "y"), multiple = T),
        uiOutput("dropdown2")
      )
    )
  )
)

server <- function(input, output, session) {
  values <- reactive({
    req(input$dropdown1)
    
    if (input$dropdown1 == "x") {
      x
    }
    
    else if (input$dropdown1 == "y") {
      y
    }
    
    else if (input$dropdown1 == "x" && input$dropdown1 == "y") {
      z
    }
  })
  
  output$dropdown2 <- renderUI({
    pickerInput("dropdown2", "Values:", choices = values())
  })
}

shinyApp(ui, server)