You can't do it for object types directly, but you can do it for class types:
  class type t1 = object method f : int end
  class type t2 = object inherit t1 method g : bool end
Defining a class type also defines the corresponding object type, which means the above results in types called t1 and t2 that are equal to the ones you want.
This trick also lets you use #t2 to refer to the polymorphic type < f : int; g : bool; .. >.
Regards,
Leo