사이: between = relation
좋다: to be good
both above combined, 사이좋다 means "to be in a good/close relation", "loving", etc
사이좋게 is the adverb from above.
A little bit more context is required to tell you the exact meaning in your case, below are my guesses though:
- Someone praying: "Please make us get closer to each other"
- The photographer telling to a couple: "arm-in-arm please"