If those two Publications are Published to the same Target, then you could use a Custom Resolver, if the Targets are different for those Publications, then I think you can only accomplish it through the Event System.
The benefits of using the Event System will be visibility, you add an additional Publish Transaction which shows up in the Publish Queue.
When using a Custom Resolver, you basically see only a single Publish Transaction in the queue, but when you inspect it, you should be able to spot the additional items which are resolved. The benefit of having everything in a single Transaction will be performance, as items inside a Publish Transaction are cached.
So you will have to make the trade-off yourself, using a Custom Resolver will probably give you better performance (depends on a lot of variables), as opposed to the Event System which might visibly make the process more clear when looking at the Publishing Queue.