Poker winnings aren't measured in a game win/loss ratio. They're measured in winrate, which can be in bb/100 or hourly.
For those who multi-table, and do it correctly, their bb/100 will typically drop as tables are added because they will player fewer hands, but their hourly rate stands to increase. As they gain proficiency at multi-tabling, they can actually think through hands, contrary to popular belief, and bring their bb/100 close to where it was with fewer tables. Especially if they use a HUD which is really a must for playing a lot of tables. I actually play worse at 1 or 2 tables most of the time because I'm bored stiff. When you get used to playing a lot of tables, there's almost no going back. Liquidity is low where I play now and getting more than 4 tables going at once is rare. I really miss playing more, because even 4 will bore me and cause me to surf or watch a video while I play. Back in the heyday of FullTilt, 12-16 tables was my norm.
When bonuses and rakeback is taken into consideration, multi-tabling will almost always be more profitable assuming if approached correctly. Even a slightly losing player can make money multi-tabling with rb and bonuses due to the sheer number of hands played per hour.