With Bryan being as much of a recluse as he was though, I dont' think the 2nd person would ever have a chance to get into the house. The house opened into the living room and Bryan would have surely locked the door immediately after letting his "guest" in. Harlow getting overally friendly with Bryan and going behind the couch to offer him a shoulder massage or something was how I pictured it happening. After that was done, he probably let his partner in the house to help finish up the task, who was probably hiding in the truck up until that point.

I doubt if the two will ever confess to a fully disclosed confession of it all, so we may never know exactly how it happened, although knowing the "why" behind it is disturbing enough.