No experience of Chapuis Doubles, but plenty on double guns.

Not opening probably due to

1) a firing pin stuck in or penetrated through a primer, thus not retracting.

2) cocking dogs stuck on knuckle - gunk or rust. Remove the fore end will determine this. But if the dogs are stuck you probably cannot get the fore end off.

3) locking bolts stuck or not withdrawing. The top lever spindals are pretty strong so doubt they would break it may be that the locking bolts are stuck - again rust or gunk.

Treat gun as if loaded. Use a cleaning rod to determine if there are cartridges in the chambers.

In the case of 1) above you might get lucky with a cleaning rod and cleaning jag provided the cartridges have fired.

Whether you are competent or comfortable using turnscrews that absolutely fit the slots only you can tell. Removing the stock and the base plate will soon allow you to determine the issue. Judicious application of easing fluids will hopefully ungunk things. And further removal of parts will allow the gun to be opened.

What I would not do is try to force it open. That will break things and that very quickly becomes expensive.

If in doubt get the services of somebody who really understands double guns to have a look.