Im trying to make a scorpion chariot for my army and was wondering if you have any ideas such as if i should use one or two tomb scorpions

Since one comes on a monster base, that equals two cav bases, so I'd say one, unless you want a really big and impressive (aka missile magnet) model

I agree, one Scorpion would look alot better than two and since they are on the same size of base, width anyway, I would just go with that. It will look cool if you can pull it off.

I think ill try to make the chariot a litlle larger than average as well sort of like arkan the blacks in 5th edition

The idea sounds pretty cool, but you should really consider the missile magnet issue- if you make it too big your opponents will claim that they can target it. No to mention that you'd seriously have to adapt the chariot profile, leading to much whining trust me on this one.