How do I keep one iPhoto library updated on two Macs without copying the entire library each time?

Sorry if this has been asked many times before, I've looked, but can't find an answer to my particular questions...

Currently I keep my iPhoto library on two different Macs by periodically copying the entire iPhoto library package from the main computer on to an external HD and then loading in iPhoto on the second machine.

The issue is that this is very time consuming and I am finding myself doing it less and less, meaning that I am often without recent photos I want access to on the second Mac. Is there an efficient way to copy only the new photos/videos added since the last copy was made on to the hard drive, that I can then add to the library on the second machine without replacing the whole library?

I wondered if it was possible to use incremental back-up software to back-up to the external drive that I could then use to update the second mac. The issue here would be that I would have to replace the whole library on the second mac each time, but at least that would be a quicker process than also having to copy the whole library from the first mac in the first place.

Or can I just select the most recent photos and videos from within iPhoto and batch export them via the external HD to the second mac? I think i read that this doesn't copy all information such as edits though? Or am I mistaken?

If none of this is possible in the way I am requiring, could anybody suggest an alternative photo storage software for Mac which I can have more control over?