Game rules: Basically, no skill in that specific weapon imo and, really, a -15 penalty - although I might do away with that since they actually do know the weapon - just in the other hand. Now if they had ambidexterity it would only be a -20 penalty off their full normal OB. However if you are using RMSS/FRP you have category ranks. Which after my next bit will show you how they make sense for weapons in a way.
Real life: I've done this as we practiced the "Princess Bride" fight at the top of the cliff for about a week each year before a local ren faire we would perform it at a couple times a day (for three days). If you do not practice it (which means ranks in the skill) , initially, I'd say it's about a 50% at best deal (think about how in RMSS/FRP you have category/stats/prof bonus, etc). You know what to do, you just aren't used to doing it with that arm. After practicing for about four days (2-3 hours a day) you get decent at it, maybe up to around 75% I'd guess, after a week maybe 90%... but keep in mind this is a choreographed fight.
Basically I think the skill setup in RMSS/FRP does a decent job of simulating this. Develop one rank in that arm and you jump from a specific skill of -15 with that weapon to +3. So, let's say it's a Fighter (Profession bonus of +20OB) with, we'll say 10 ranks in category (+20) and an average stat bonus of +6. That's +58OB without outside factors from category alone. Now, he'll lose 15 points due to no skill, so you have 43OB. Buy one rank in your off-hand and you have +61OB. But like I said above, I'd probably do away with the -15 as a house rule (so long as they are 'highly' trained in that weapon in the other hand). The reason for this is it's not like they know nothing about using that weapon, they are just using it with the other hand.