I want to know if T491A104K035AT and T491A104K050AT are substitutes for each other.
It depends on your application.
As Barley stated, the only difference between the two is voltage. So, one can ALWAYS substitute the 50V 491A104K050AT for the 35V T491A104K035AT, but the reverse is not true. The 35V part could only be substituted if the application voltage is less than 18V.
When using standard tantalum capacitors such as these, one should not place them in circuits with steady-state voltages of more than about 1/2 of their rated voltage. So, for the 35V part, it should not be used in circuits with steady-state voltage above about 17.5V, and for the 50V part, steady-state voltage should not exceed 25V.