I have a retaining wall separating the neighbor back yard to my garage. The retaining wall is about 7ft high and is made of solid concrete. The retaining wall attaches perpendicular with my neighbors retaining wall but I noticed retaining wall has about a 6" gap were the two retaining walls meet up. Does anyone have any solutions. I was thinking of reattaching the two retaining walls with a metal joint or in the extreme case drilling a hole in my retaining wall and anchoring it in by cementing rebar to a concrete cylinder if my neighbor would let me get a post hole digger in the back yard.

I also live in Calgary so thaw and de thaw is a concern.

I have a good bit of retaining wall experience but this is a big challenge from your description. I am betting that you have had a lot of rain (snow). Is there relief for water collecting behind the wall? Depending on the type of design, the footing might be settling.
The only remedy I can think of is to dig behind the wall to relieve the soil load (I know-no access), and possibly pump grout under the front of the footing. These are usually poor drainage problems. Depending again on the design and length, tieing to the other perpend. Wall might delay it going further. This is a serious problem. Water trapped behind the wall will destroy it.