Tiller motor steering

Our 90 horse will only troll around 1.9+. Like to get that down some, we have a 20 horse tiller just for the sake of safety, if we start the tiller and set the throttle can we steer with the 90? Kind of like a rudder? Thanks guys

I do this with my electric motor on front, pointing straight, and my gas motor on back as tiller.  I don't use my boat in Erie, but works ok on inland lakes.  It won't steer as true as your 90, but should rudder it a bit.  It doesn't work very well in a stiff breeze though.  I wouldn't use if going more than 1-2 mph (like using your 20 as your drive on a restricted lake), or if you are fishing with a bunch of company on the big lake or trying to stay on an exact structure and need precise control.
 
A better option is using an auxiliary motor connector rod.  I have a 15 tied into my 50 with one, so I can use the 50 control wheel for my 15.  I bought one from Cabela's, for $30 about 6 years ago, and still works fine.  Make sure to disconnect from your main motor when using the big one at high end speeds.  I use mine on my 15 at full throttle with no issues.  Granted, that is 10 mph without company and 7 mph fully loaded...  Hard left hand turns are a bit rough at slow speeds, since the auxilary motor is not mounted in the center of the boat, but most control is pretty accurate.

Here's my old boat set-up with a tie-bar mounted to the front of both motors. 
 

 
As mentioned here before, you can buy the kit for around $50 bucks.
 
You might need to fabricate a bracket to make the unit line up properly to both motors.  I lucked out on my last boat since both motors lined up pretty evenly and I was able to run just the straight rod. 
 
Sometimes you might be faced with putting a bend in the rod to get the rod to line up.