How do I paste a column of text after a different column of text in Vim?

I have two columns full of text, and I want to get them side-by-side. For example, I have

abc
def
ghi

and

123
456
789

and I want

123 abc
456 def
789 ghi

All I can find is how to paste the same line onto the end of every line in a column. I can't figure out how to paste a multi-line block of text on the end of another column.