Sorry but there are no lefty Japanese razors. They are all the same. Yes you can have someone custom make you a blade similar but it would not be a Japanese blade it would be a symmetric western styled blade but with a fixed design so it looks like a traditional japanese straight but it would not be one.

As far as using both sides of the razor, yes you can do that and it will work but sorry you don't get the bragging rights to say you mastered a japanese razor. Like learning to ride a bike with training wheels on and never taking them off. If you look at the iwasaki Razor you will notice it isn't just that the razor is asymmetric but the entire blade edge is canted towards the concave side which it the side that should remain to your face at all times. I'm not sure if the Tosuke is the same but that will affect using the razor from the convex side. Also if you sharpen one the same way you sharpen a western style razor you are slowly changing the symmetry of the thing.

They are different and require different techniques for honing and shaving. That's what makes them different, challenging and fun to use. Just another dynamic in learning to use different kinds of straights. If you take the time to properly learn you will be rewarded with the best possible shave you wil ever get.