1. Install a 6.1.20 EE Liferay server with the default patching tool (v5).
2. Add a hotfix (e.g.: liferay-hotfix-784-6120.zip) (It is not indifferent which one!)
3. Upgrade your patching tool to v11.
4. Add an other hotfix (e.g.: liferay-hotfix-904-6120.zip).
5. Install the patches.
There will be an error: "Error: portal-impl/build.xml"
Workaround: downgrade your patching tool to v5, revert, upgrade to v11, then install.