diff --git a/rs/justfile b/rs/justfile
index 78834941d2..8074f2f9d4 100644
--- a/rs/justfile
+++ b/rs/justfile
@@ -220,9 +220,10 @@ _select $FILES:
     ' <<< "$metadata")
 
     # A crate has to be rebuilt when anything it depends on changed, so walk the
-    # edges backwards until the selection stops growing.
-    selected=$(awk -v seeds="$seeds" '
-    	BEGIN { split(seeds, s, "\n"); for (i in s) want[s[i]] = 1 }
+    # edges backwards until the selection stops growing. The seeds go through the
+    # environment because BSD awk (macOS) rejects a newline in a `-v` value.
+    selected=$(SEEDS="$seeds" awk '
+    	BEGIN { split(ENVIRON["SEEDS"], s, "\n"); for (i in s) want[s[i]] = 1 }
     	{ pkg[NR] = $1; dep[NR] = $2 }
     	END {
     		do {
@@ -409,6 +410,11 @@ _select-test:
     	fail "a moq-relay diff must not drive the media contract: $(just rs _names "$relay")"
     fi
 
+    # Several seeds select all of them, not just the first.
+    pair=$(just rs _names "$(just rs _select $'rs/moq-auth/src/lib.rs\nrs/moq-hls/src/lib.rs')")
+    grep -qw moq-auth <<< "$pair" && grep -qw moq-hls <<< "$pair" \
+    	|| fail "a two-crate diff must select both: $pair"
+
     # A crate nested inside another crate's directory is covered by its parent's
     # seed. Selecting it directly would compile libFuzzer on every moq-net diff,
     # and its directory name is not its crate name, so it would also mis-report.