I think there are a multitude of causes for excessive or repetitive licking. I don't personally feel in general it is a manipulative behavior on the dog's part. I think to some dogs it just becomes soothing. My Tommy will often lick on one foot or the pads of the foot slowly and repetitively when he is settling down to relax for a long while. He will do it in another room or the room I'm in, whether I say anything to him or not. He often does it for awhile before goes to sleep, it reminds me of thumb sucking.

Tasha is cat-like in her grooming and extremely cleanly. She will spend a lot of time cleaning her paws and legs, and even biting her nails down. I've even seen her grooming on my other two.

I think it can just simply become a habit. Perhaps in the case of the Pekes who both do so, one took the cue from the other and so it is something they both do now.

For me if it gets to be annoying I will simply say their name and they stop.