Is it possible to go to Dushanbe from Samarkand with share taxi

It's doable but you will likely need to take more than one vehicle--one for the short trip to the border, and then from there either to Samarkand (if you're lucky) or, if there are no other passengers, to Denau or somewhere farther along your way.

It's certainly doable in a day from Dushanbe. I can't really help with costs, and in any case these will depend on how many others are sharing the ride assuming you don't want to wait for a vehicle to fill up.

By a direct one? No but as indicated you can use few of them and reach Dushambe.
Be carefull because all the border posts between the two countries are not open...

Michel

Once over the border, the taxi drivers in the small town are very aware that your choices of transportation are very limited. Bargain hard and get a shared taxi with locals to join you.