Because liquid will seek to be level; the bottom of both tanks will have to be mounted at the same height. The top of the tanks should be the same height of each other or you will only be able to fill the lower height tank. This is because the fuel will run from one tank to the other and as one tank is (for example) filled 4 " from the bottom so will the second tank. The vented caps should be fine.
Your working fuel gauge will read OK, but remember that when it reads 1/2 you will have half in one tank and half in the other.
Using a 10 gallon tank and an 11 gallon tank will not enable you to have a total of 21 gallons unless the tanks are both the same height.