No, you cannot connect both Netsuite accounts to the same FarApp account. Only one account can be connected to a FarApp account at one time. The Netsuite sandbox account will require you to create a FarApp sandbox account. To set up a FarApp sandbox account, please refer to this article: https://support.farapp.com/hc/en-us/articles/115011564887-Quick-Reference-6-Setting-up-a-FarApp-Sandbox-Account.
Note: FarApp strongly recommends that you create separate accounts for your sandbox and production accounts. However, you can connect your Netsuite sandbox account to your FarApp account and then later switch it to your Netsuite production account by changing the credentials in FarApp. This approach is against leading practice since it will purge all of your data (i.e. imported orders, products, etc.). And you will need to adjust your mappings and settings to conform with the configuration in your Netsuite production account. To learn more please see this article: https://support.farapp.com/hc/en-us/articles/360036152414.