I will usually recommend going with a good 1/2HP or even 3/4HP sump pump as your second "primary" pump, just in case you need extra power during a major storm. There are just a few things you need to understand.
1. Does your current sump basin have enough room? A standard sump basin that measures 18" wide by 24" deep will have sufficient room for two pumps.
2. Do you have a separate dedicated 120 volt outlet for that second pump? All sump pumps should run on a dedicated circuit.
3. Make sure that the float switch for the second pump is raised a good 2"-5" above the activation point of your "Main" pump.
4. How do you plan on discharging the water from that second pump? If at all possible, it's always best to run a separate discharge pipe for the second pump. This will ensure that you will get the optimal pumping capacity from BOTH pumps should you need them during a major storm.
Of course you should also consider the possibility of power outages as well as the need for a battery backup system or small standby generator.