I got the idea for using a thrust bearing from the 4th page of this
thread
In terms of strength I was referring to the support between the two wheels.
Also, one thing to remember is that you will want to build and test this before the season starts if you want to use it this season. You do not want to try to do an untested drive during the season. Trust me, my team did that this year and we did not have the best time, but after the season we tweaked it and it works great.