How do I sync photos onto my ipad from multiple computers?

My dad just got an ipad and we're trying to sync pictures from multiple computers. He's already synced pictures from his computer. But there are some pictures on my computer that he wants too. Every time I try to sync photos from my comptuer to his ipad, a menu pops up and asks if I want to replace his existing photos. 😕 I don't want to replace his photos with my photos. I just simply want to add to his collection of photos.

Of course I can always transfer photos on my computer onto a flash drive and upload it onto his computer. But I want to know if there is a way where I can sync photos from multiple computers onto his ipad without replacing his original pictures.. (and without using a flash drive or emailing pictures)

Thanks😀