void foo<TOne, TTwo>() 
   where TOne : BaseOne
   where TTwo : BaseTwo

More info here:
http://msdn.microsoft.com/en-us/library/d5x73970.aspx

void foo<TOne, TTwo>() 
   where TOne : BaseOne
   where TTwo : BaseTwo

More info here:
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters#constraining-multiple-parameters