I thought you had to have them in pairs because that's the only way I've seen them in various loadout-charts. If one is enough then I guess that helps a bit.
I'm somewhat concered in general about the limited loadout of the Viggen though - I mean, I hate to be mr. negative here and rain on everyones parade, but it seems to me that the payload will be very limited if you want some basic counter-measures as well as some basic anti-air capability. Because then you're basically left with a total air to ground payload of 2 or 3 mavericks (or whatever you choose), which is hardly very impressive for an aircraft that is supposed to be a ground pounder. It's also made even worse by the fact that it doesn't have any cannon either.
Don't get me wrong though, I'm still SUPER EXCITED about this module and I will buy it as soon as I have a chance. I just wish that the swedish airforce back in the 60's would have decided to give the Viggen the capability to carry a larger payload - I mean it seems like weight is not the problem since it can carry 3 anti-ship missiles, so all they would have needed to do was to buy some of those bi-couplers for mavericks so you can carry 2 per pylon, and get some bigger bombs etc. but no... stupid 60's aircraft designers - didn't they think about gaming in 2016 at all? :)