The gunnels on the wherry are so low that there is little room between the oar handles and your legs during the stroke recovery when set up for fixed seat. There is not enough angle to get the blades out of the water. With the drop in sliding seat the oarlocks are a couple inches higher and farther outboard. This provides the proper oar angle. Even so you will still clip wave tops with the blades when it gets rough. I am talking sculling oars here. (9'6" inches in my case) Sculling oar also generate LOTS of power which would be transmitted directly through the oarlocks to the gunnels. Not sure I like that idea. You might be able to compensate by using shorter oars. But then you would sacrifice a lot of speed. I'm pretty sure the tandem is just a longer version of the single so you would have the same issues x 2. In my opinion the wherry is not the best choice for fixed seat rowing.

I am 5' 8"  155 lbs,  I routinely use my wherry as a fixed seat.  It has been mentioned before, (I asked), but you have to install some kind of foot rest between the seats if you are going to fixed seat row.  

 Having said all that,  I use 7 foot oars and have no problem on the stroke,  I feather my oars and have found I barely kiss the water or do not touch at all on my return stroke.

On topic, (kinda) what I did find suprising is the Ebenezer Geosidic Ultrilight whitehall I finished up is also a great joy to row.  The ultralight has a very small foot rest built into the design.  And found it was big enough, so I installed a fairly small one in the wherry,  just big enough for my heal of my shoes to rest (or the flip flops more often then not).

 Hope above helps