You don’t get to specify a duration of validity on the standard form. The only dates you submit are those of your first trip to the Schengen area. What you can do is check the box asking for a multiple-entry visa and provide evidence of your need to travel to the Schengen area regularly. If you have a very specific and very convincing reason to want a two-year visa (as opposed to the generic flexibility afforded by a multiple-entry visa), you could write a letter to explain that and attach it to your application but there isn’t really a way to apply for a two-year visa. Depending on your personal situation, it would be unlikely to get that on a first application and the dates of validity of your visa are still entirely up to the consulate.
If granted, a multiple-entry visa would go beyond the dates of your first trip and typically be valid for at least 6 months. The recommandations of the Handbook are not always followed but the EU Commission recommends issuing a one-year visa first, to be increased to two and then three years on renewal. UK residents sometimes get five-year visas on the first application, even without asking for it, but that doesn’t help you if you reside in Nigeria.