I'm very sorry to hear about your dog :( Either shoulder blade will have enough room for two, if you want it that small. The shoulder has long been an intimate place to put memorials and have enough space.
But, if I may suggest one thing? If I were you, I wouldn't get anything. Please do not take this the wrong way, as I'm heavily tattooed myself, but I've seen thinkers like you take 6-10 years to get something only to turn around and regret it merely days later, no matter what it was. If you have to think this long and hard on a tattoo, I don't think it was meant to be.
Again, please do not take this the wrong way, I just don't want you to regret putting something on your body, ok? My brother keeps saying, I'm getting one, I'm getting one...nothing yet in 8 years. And my sister finally bit the bullet after 10 years of thinking about it and HATES hers. I just don't want you to waste money or anything and there are plenty of other ways to remember your doggie, other than your body.