On first glance, the dual role would seem to be a difficult one to carry off. There's a good reason most CFL teams tend to have separate coaches and general managers; coaching a team is a full-time job in and of itself, and CFL personnel duties aren't the easiest, as personnel departments have to simultaneously keep track of their active, injured and practice rosters, who might be available from other CFL teams, what former NCAA or CIS players might be out there, who the latest NFL cuts are, and who's playing in other leagues like arena football, the UFL or some of the European ones. They also have to keep on top of which players are on whose negotiation list, while following CIS football and all levels of NCAA football to prepare for the annual Canadian draft and the selection of former American college players.
The CFL's a league that brings in players from a wide variety of sources, and it takes a lot of time and effort to keep track of all of those potential sources. It's also important to note that the differences in the Canadian game mean that personnel executives have to try and envision how Americans will fit in up north; keep in mind that Cam Wake, one of the best defensive ends in the league in recent years, was a linebacker both in college and the NFL. When you add all that up, it's clear it takes a substantial amount of time and effort to manage the personnel side of a CFL team, and it's effort that goes on throughout the season (especially with NFL cuts), not just before the year starts. It can be tough to find the time to give that side of the game its due if you're preoccupied with head coaching responsiblities.
However, it's worth noting that the head coach/general manager divide perhaps isn't as significant as it seems on first glance. Even the CFL coaches who don't carry the GM tag tend to have a good amount of input into personnel decisions, which is as it should be; if an organization's personnel don't fit the coach's schemes, the team probably isn't going to find a lot of success. The personnel and coaching sides have to be reasonably unified regardless of if they're led by different individuals or not.
Moreover, giving one man both the head coach and general manager titles doesn't mean they're necessarily doing the work of two people. For example, Hufnagel carries both titles with the Stampeders, but their football operations department also includes director of player personnel John Murphy, director of football operations Michael Petrie, defensive coordinator and assistant director of player personnel and football operations Chris Jones among others. It's impossible to know exactly how decisions are made from the outside, but I'd venture that Hufnagel likely oversees and coordinates the work of both his assistant coaches and his personnel men. The dual title puts him at the clear top of the organizational pyramid, but he has a significant number of assistants to help out on both the personnel and coaching fronts.
By contrast, the Winnipeg football operations staff seems smaller. Vice president and general manager of football operations Joe Mack, assistant general manager and director of football operations Ross Hodgkinson and director of player personnel Ken Moll appear to be the only people there on the personnel side (I'd imagine head coach Paul LaPolice also has some input into personnel decisions). Yet, if you add the positions up, their department has just as much manpower devoted to personnel as Calgary; if Murphy and Petrie are treated as both full-time personnel and Hufnagel and Jones are treated as half-time coaching, half-time personnel, that's a total of three full-time personnel jobs, identical to Winnipeg. This isn't intended as an exact comparison of man-hours each organization devotes to personnel, as that would be very tough to do from the outside, but rather just a note that titles aren't always everything and combining the top jobs doesn't necessarily mean reducing the number of front-office people.
There are both significant advantages and pitfalls to uniting or separating the top jobs. If you combine the top titles, you've got a clear organizational structure and an obvious overarching mould for the team, both of which can be very good things; just look at what Buono and Hufnagel have accomplished in their tenures in B.C. and Calgary. You lose some of the checks and balances and different viewpoints that you might get with different coaches and general managers, though. It obviously wouldn't work to have a coach and general manager that don't think along the same lines at all, but coaches and general managers with slightly different ideas can build off each other and develop a best of both worlds approach.
The other danger with uniting the top jobs is that it definitely presents the chance for increased burnout and health issues (which can be a significant factor in the coaching profession; just look at Dick Vermeil, Urban Meyer or Don Matthews). It would be easy to overwork yourself as either a coach or a general manager; with both titles, the potential for burnout definitely increases. Succesful holders of both titles need to have the ability to delegate and they need to have a strong cast of assistants to delegate to; significant experience at the CFL level probably also helps with knowing what they need to do themselves and what they can pass off to others, which is certainly the case with Buono and Barker and is getting to be the case with Hufnagel. We'll have to wait and see if this move works out for Barker and the Argonauts, but it's worthwhile to remember that CFL coaching and personnel departments need to be judged as a unit, not just the man or men at the top.