a.wat

(module
 (import "" "f"  (func $f (param $n i32) (result i32)))
 (func $_start (export "_start") (result i32)
   i32.const 0
   call $f
 )
)

b.wat

(module
 (func $f (export "f") (param $n i32) (result i32)
   (i32.add (local.get $n) (i32.const 1))
 )
)

link.sh

wat2wasm --debug-names --relocatable a.wat
wat2wasm --debug-names --relocatable b.wat
wasm-ld-11 -o main.wasm a.wasm b.wasm