for loop to stack rasters stored in different lists in R

I have a nested List with many elements and x number of rasters inside each element. The simplified structure of the list looks like this:

## create main List
r <- raster(ncol=3, nrow=3)
values(r) <- 1:ncell(r)
s <- stack(r,r*2,r*3)

List <- split(as.list(s), 1)  # here List has a single element but in the real example has >10

I need to stack rasters stored in another list (named Ref_List) to the rasters stored in the main List, accordingly:

stack(List[[1]][[1]], Ref_list[[1]])
stack(List[[1]][[2]], Ref_list[[2]])
stack(List[[1]][[3]], Ref_list[[3]])

...

The number of rasters contained in Ref_list[[i]] are the same as contained in List[[i]][[j]]. I have tried the following loop to perform the stack operation but the result is NULL.

Any ideas about how to solve this issue? Thanks!

List_stack <- lapply(List,function(i){ # i specifies elements within the list

  lapply(i,function(j){  # j specifies the elements of each element, within the list

    st <- stack(j)

    ## create REF list
    z <- raster(ncol=3, nrow=3)
    values(z) <- 0.5:ncell(z)
    t <- stack(z,z*2,z*3)
    Ref_list <- as.list(t)

    ## STACK REF list with MainList
    for(i in 1:length(Ref_list)){
      final <- stack(Ref_list[[i]], st)
    }
   }) 
})