[yorkieusa]

I wouldn't try to give equal attention. I would, especially, give Sioux, and Dakota, too, more attention. Give them more attention, so they know they aren't being thrust into the background. It was their home first. They come first. Give them their food and treats first, as well. Have you tried walking Sioux and Spencer together to see if you can help them to bond?

Sometimes these things take care of themselves after a few weeks or longer. Sometimes it gets worse. I hope things work out for you.