diff --git a/taca/element/Element_Runs.py b/taca/element/Element_Runs.py
index c0c2c0c2..a338d691 100644
--- a/taca/element/Element_Runs.py
+++ b/taca/element/Element_Runs.py
@@ -7,6 +7,8 @@
 from datetime import datetime
 from glob import glob
 
+import pandas as pd
+
 from taca.utils import misc
 from taca.utils.filesystem import chdir
 from taca.utils.statusdb import ElementRunsConnection
@@ -174,6 +176,9 @@ def get_lims_step_id(self) -> str | None:
         """If the run was started using a LIMS-generated manifest,
         the ID of the LIMS step can be extracted from it.
         """
+
+        # TODO test me
+
         assert self.manifest_exists(), "Run manifest not found"
         with open(self.run_manifest_file_from_instrument) as csv_file:
             manifest_lines = csv_file.readlines()
@@ -186,6 +191,8 @@ def get_lims_step_id(self) -> str | None:
     def copy_manifests(self) -> bool:
         """Fetch the LIMS-generated run manifests from ngi-nas-ns and unzip them into a run subdir."""
 
+        # TODO test me
+
         # Specify dir in which LIMS drop the manifest zip files
         dir_to_search = os.path.join(
             self.CONFIG.get("Aviti").get(
@@ -244,6 +251,114 @@ def copy_manifests(self) -> bool:
 
         return True
 
+    def make_demux_manifests(
+        self, manifest_to_split: os.PathLike, outdir: os.PathLike | None = None
+    ) -> list[os.PathLike]:
+        """Derive composite demultiplexing manifests (grouped by index duplicity and lengths)
+        from a single information-rich manifest.
+        """
+
+        # TODO test me
+
+        # Read specified manifest
+        with open(manifest_to_split) as f:
+            manifest_contents = f.read()
+
+        # Get '[SAMPLES]' section
+        split_contents = "[SAMPLES]".split(manifest_contents)
+        assert (
+            len(split_contents) == 2
+        ), f"Could not split sample rows out of manifest {manifest_contents}"
+        sample_section = split_contents[1].split("\n")
+
+        # Split into header and rows
+        header = sample_section[0]
+        sample_rows = sample_section[1:]
+
+        # Convert to list of dicts
+        sample_dicts = []
+        for row in sample_rows:
+            row_dict = dict(zip(header.split(","), row.split(",")))
+            sample_dicts.append(row_dict)
+
+        # Convert to dataframe
+        df = pd.DataFrame.from_dict(sample_dicts)
+
+        # Separate samples from controls
+        df_samples = df[df["Project"] != "Control"].copy()
+        df_controls = df[df["Project"] == "Control"].copy()
+
+        # Apply default dir path for output
+        if outdir is None:
+            outdir = self.run_dir
+
+        ## Build composite manifests
+
+        manifest_root_name = f"{self.NGI_run_id}_demux"
+
+        # Get idx lengths for calculations
+        df_samples.loc[:, "len_idx1"] = df["Index1"].apply(len)
+        df_samples.loc[:, "len_idx2"] = df["Index2"].apply(len)
+
+        # Break down by index lengths and lane, creating composite manifests
+        manifests = []
+        n = 0
+        for (len_idx1, len_idx2, lane), group in df_samples.groupby(
+            ["len_idx1", "len_idx2", "Lane"]
+        ):
+            file_name = f"{manifest_root_name}_{n}.csv"
+            runValues_section = "\n".join(
+                [
+                    "[RUNVALUES]",
+                    "KeyName, Value",
+                    f'manifest_file, "{file_name}"',
+                    f"manifest_group, {n+1}/{len(df.groupby(['len_idx1', 'len_idx2', 'Lane']))}",
+                    f"grouped_by, len_idx1:{len_idx1} len_idx2:{len_idx2} lane:{lane}",
+                ]
+            )
+
+            settings_section = "\n".join(
+                [
+                    "[SETTINGS]",
+                    "SettingName, Value",
+                ]
+            )
+
+            # Add PhiX stratified by index length
+            if group["phix_loaded"].any():
+                # Subset controls by lane
+                group_controls = df_controls[df_controls["Lane"] == lane].copy()
+
+                # Trim PhiX indexes to match group
+                group_controls.loc[:, "Index1"] = group_controls.loc[:, "Index1"].apply(
+                    lambda x: x[:len_idx1]
+                )
+                group_controls.loc[:, "Index2"] = group_controls.loc[:, "Index2"].apply(
+                    lambda x: x[:len_idx2]
+                )
+
+                # Add PhiX to group
+                group = pd.concat([group, group_controls], axis=0, ignore_index=True)
+
+            samples_section = (
+                f"[SAMPLES]\n{group.iloc[:, 0:6].to_csv(index=None, header=True)}"
+            )
+
+            manifest_contents = "\n\n".join(
+                [runValues_section, settings_section, samples_section]
+            )
+
+            file_path = os.path.join(outdir, file_name)
+            manifests.append((file_path, manifest_contents))
+            n += 1
+
+        for manifest_path, manifest_contents in manifests:
+            with open(os.path.join(outdir, manifest_path), "w") as f:
+                f.write(manifest_contents)
+
+        manifest_paths = [t[0] for t in manifests]
+        return manifest_paths
+
     def generate_demux_command(self, run_manifest, demux_dir):
         command = [
             self.CONFIG.get(self.software)[