You will be able to configure project links if this setting is enabled in your login profile > setup permissions.
On the Project Links screen, you can allocate up to 100% of the costs of the current project to one or more other projects.
Alternatively, a link with an allocation of 0% can simply signify a relationship or dependency between the two entities, with no financial implications.
To add a new link, select a project from the dropdown list and click the add button. If there are no existing links, the default allocation will be 100%, otherwise the default allocation will be 100% minus the sum of the existing allocations.
If a project has links from other projects these will be shown in a separate read-only table beneath.
Links can only be made at the lowest level of the hierarchy, i.e. project to project and this is to ensure values roll-up through the project hierarchy without double-counting.