I think the any problem will come once Walker reaches sexual maturity. The way it's been talked about in various programmes I got them impression that Walker won't be left in with Mercedes (or another female) full time once he reaches that point. At the moment it's fine because he's just young.
The fencing in the current enclosure will need to be strengthened to take an adult male, but the keeper said by that point Walker's new enclosure will be ready.
There are two outdoor bits attached to the house bit, but only one house bit. I took a couple of photos the last time I was up, but they aren't brilliant. The bit Walker is in is uncovered, the bit he is trying to get to was the bit Mercedes was in and is covered.
The main enclosure area is on the left as you look at the photo and they have access to the area on the right that used to be part of the Wolf area. This has a small shelter area, but I couldn't tell you if it was lockable or suitable to stay at night in or anything.
Perhaps the problem with keeping them together in the current enclosure (aside from Walker attempting to mate Mercedes) could be the lack of house space and it would also make it difficult to separate them if need be? I get the impression that when they end up with Walker and the female they will be kept separately except for during mating season.
Although I do get the impression that they don't expect Mercedes to still be alive when the time comes for finding Walker a mate. You can really see how she ambles along and moves with some difficulty since seeing her next to Walker.