The National Standard of Canada, Installation Code for Solid Fuel Burning Appliances
and Equipment, CSA-B365-01 clause 5.2.9 states that:
Two or more appliances shall not be connected to the same chimney flue unless:
1) The appliances are located on the same story;
2) A negative pressure is maintained at the flue collar of each appliance;
3) Each appliance is provided with a means to control the rate of flow of air or flue
gases through the appliance combustion and heat exchanger chambers. Where
limitations on flue pressure (draft) are contained in the manufacturer’s instructions,
such means shall be capable of maintaining at the flue collar, the pressure
specified by the appliance manufacture; NOTE: For the purpose of this Clause,
draft controls include ash pit air control dampers, dampers or draft slides wherever
located, over fire and flue key pipe dampers, and check draft controls located on or
in the flue pipe.
4) The flue pipes of the appliances are connected to a manifold as close to the
chimney as practicable or directly to the chimney in the following order:
(i) If only solid fuel is used, the flue pipe from the smallest appliance shall be on
(ii) If different fuels are used as permitted in Clause 5.2.7, the oil flue pipe shall be
on top (downstream); and the solid fuel flue pipe on the bottom (upstream); and
5) The chimney flue is capable of venting the flue gas by natural draft when all
appliances not interlocked to prevent simultaneous firing are firing at their
maximum rate at the same time.
When installing a wood-burning appliance to an existing chimney carefully inspect entire
chimney for the presence of old inlet holes, which may be improperly covered by metal
caps or other unacceptable means. Fill any openings with brick and mortar to ensure
no hazardous openings exist.
Judging from the size of the gas insert. It is possible for this to be completely separate. However I would not want to call that safe as you would need to anchor to the original chimney and that is not accepted. You also need proper brackets all the way up to the top anchored somewhere.