Hope this helps.
1) Does the second battery in the bank have to be the AMP rating at the first?
No, but it must be the same type e.g. if your existing battery is SLA (sealed lead acid) then you should use SLA. If your existing is Gel then you would need a Gel etc. Depending on the age of your existing battery it is sometimes good practice to replace both. Why not look at a couple of decent traction batteries since you are looking at changing.
2) When connecting the two batteries what gauge cable should I use?
Depends what you going to use them for. If you are using an inverter then the cable should be suitable for the load. If you are just running standard equipment then look at the existing cable thickness. If it is 6mm then use 6mm (assuming the batteries are to be co- located), If the existing is 16mm then use 16mm etc.
3) When connecting the solar charge controller to the battery bank am I correct in thinking that you connect the positive to one of the batteries and the negative to the other to ensure even charging?
It is good practice, but not essential for two batteries as long as you maintain a parallel and not serial connection between the two assuming 12 volts)
4) The MH
battery charger is currently connected to one battery do I need to connect this across the two batteries as mentioned in point 3 above to ensure proper charging?
Again good practice, but not essential for two batteries.