Generally when you take a DHT inhibitor it causes an abrupt change to your hormonal levels which can trigger a temporary and sometimes aggressive form of hair loss called telogen effluvium. This is the initial shed you hear many people speak of when starting these treatments. Most of the time it's an acute form which usually starts between a month to three months after starting your treatment. This form of hair loss lasts between two to four months and by then all forms of hair loss should cease. If you are still shedding heavily after that point it's possible that the treatment is causing a chronic form of Telogen Effluvium which means you are not a responder to that treatment. Continuing to use will only accelerate and not help your hail loss situation.
In regards to stacking Dutasteride and Finasteride. If you are taking the daily recommended dose of Dutasteride there shouldn't be enough 5AR in your system for Finasteride to bind to. You can however, add a lower dose of Dutasteride such as one or two pills a week for an additional boost. Anything more than 3 doses of Dutasteride a week will then push it over Finasteride's efficacy rendering it useless to even take.