Builders use wall ties in cavity walls to hold the two walls together and stop them bulging or bowing. In old properties it is common to find steel wall ties - these rust and corrode and have to be replaced. Modern wall ties are made from either stainless steel or galvanised steel, these have a much longer lifespan as they do not rust.
Problems with old wall ties can be picked up on a structural survey - a hole is drilled into the wall and a camera is used to survey the cavity. Fortunately there are companies who specialise in this work, as this issue can affect your mortgage decision.
Some older buildings have pieces of slate to tie the two structures together; again, this is not acceptable under modern building regulations, so they will have to be replaced.
Replacing wall ties is usually done from inside the house - it involves drilling through the inner brick and into the second brick. They then inject a layer of resin, insert the stainless steel wall tie, then inject more resin.
Alternatively, it is possible to use threaded stainless steel rods with special rubber grippers on each end. Once they are inserted you can use a cordless drill to spin the nut on the end and tighten the ends - this forces the rod to grip both the inner and outer wall.
With new builds this is much easier - you simply embed the ties into the mortar at specified intervals.