diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index f9e3c92..befa9ea 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -1,6 +1,6 @@
 repos:
 - repo: https://github.com/pre-commit/pre-commit-hooks
-  rev: v4.1.0
+  rev: v4.5.0
   hooks:
   - id: trailing-whitespace
     args: [--markdown-linebreak-ext=md]
@@ -8,7 +8,7 @@ repos:
   - id: check-yaml
   # ----- Python formatting -----
 - repo: https://github.com/charliermarsh/ruff-pre-commit
-  rev: v0.1.13
+  rev: v0.3.2
   hooks:
       # Run ruff linter.
   - id: ruff
@@ -24,10 +24,10 @@ repos:
   - id: pretty-format-yaml
     args: [--autofix, --indent, '2']
 - repo: https://github.com/tox-dev/pyproject-fmt
-  rev: 1.6.0
+  rev: 1.7.0
   hooks:
   - id: pyproject-fmt
 - repo: https://github.com/abravalheri/validate-pyproject
-  rev: v0.15
+  rev: v0.16
   hooks:
   - id: validate-pyproject
diff --git a/pydeps2env/environment.py b/pydeps2env/environment.py
index 216a3b3..b6a1c42 100644
--- a/pydeps2env/environment.py
+++ b/pydeps2env/environment.py
@@ -150,3 +150,11 @@ def export(
                 )
             with open(p, "w") as outfile:
                 yaml.dump(conda_env, outfile, default_flow_style=False)
+
+    def combine(self, other: Environment):
+        """Merge other Environment requirements into this Environment."""
+        self.requirements = combine_requirements(self.requirements, other.requirements)
+        self.build_system = combine_requirements(self.build_system, other.build_system)
+        self.pip_packages = list(set(self.pip_packages) | set(other.pip_packages))
+        self.pip_packages.sort(key=str.lower)
+
diff --git a/pydeps2env/generate_environment.py b/pydeps2env/generate_environment.py
index 7d6ea40..d26cd8f 100644
--- a/pydeps2env/generate_environment.py
+++ b/pydeps2env/generate_environment.py
@@ -4,7 +4,7 @@
 
 
 def create_environment_file(
-    filename: str,
+    filename: list[str],
     output_file: str,
     channels: list[str],
     extras: list[str],
@@ -16,7 +16,9 @@ def create_environment_file(
     except ModuleNotFoundError:  # try local file if not installed
         from environment import Environment
 
-    env = Environment(filename, pip_packages=pip, extras=extras, channels=channels)
+    env = Environment(filename[0], pip_packages=pip, extras=extras, channels=channels)
+    for f in filename[1:]:
+        env.combine(Environment(f))
 
     _include = include_build_system == "include"
     env.export(output_file, include_build_system=_include)
@@ -27,9 +29,9 @@ def main():
 
     parser = argparse.ArgumentParser()
     parser.add_argument(
-        "setup", type=str, default="pyproject.toml", help="dependency file"
+        "setup", type=str, nargs="*", default="pyproject.toml", help="dependency file"
     )
-    parser.add_argument("env", type=str, default="environment.yml", help="output file")
+    parser.add_argument("-o", "--output", type=str, default="environment.yml", help="output file")
     parser.add_argument("-c", "--channels", type=str, nargs="*", default=["defaults"])
     parser.add_argument("-e", "--extras", type=str, nargs="*", default=[])
     parser.add_argument(
@@ -42,12 +44,13 @@ def main():
     parser.add_argument("-p", "--pip", type=str, nargs="*", default=[])
     args = parser.parse_args()
 
-    if not Path(args.setup).is_file():
-        raise FileNotFoundError(f"Could not find file {args.setup}")
+    for file in args.setup:
+        if not Path(file).is_file():
+            raise FileNotFoundError(f"Could not find file {args.setup}")
 
     create_environment_file(
         filename=args.setup,
-        output_file=args.env,
+        output_file=args.output,
         channels=args.channels,
         extras=args.extras,
         pip=args.pip,
diff --git a/test/pyproject.toml b/test/pyproject.toml
index a27d8b8..fcc2844 100644
--- a/test/pyproject.toml
+++ b/test/pyproject.toml
@@ -11,6 +11,7 @@ dependencies = [
   "IPython",
   "numpy>=1.20",
   "pandas>=1",
+  "both>=1",
 ]
 [project.optional-dependencies]
 pip_only = [
diff --git a/test/setup.cfg b/test/setup.cfg
index ed4bbb1..3239ba5 100644
--- a/test/setup.cfg
+++ b/test/setup.cfg
@@ -12,6 +12,8 @@ setup_requires =
 install_requires =
     numpy >=1.20
     pandas >=1.0
+    both <=3.0
+    setup_only >=1.0
 
 [options.extras_require]
 test =