The cantrip mind sliver from Tasha’s Cauldron of Everything (p. 108) has a secondary effect that subtracts 1d4 from the target’s next saving throw:
(…) subtract 1d4 from the next saving throw it makes before the end of your next turn.
The spell bane from the Player’s Handbook (p. 216) has a similar effect:
(…) the target must roll a d4 and subtract the number rolled from the attack roll or saving throw.
Does the saving throw penalty from mind sliver stack with the effect of bane, causing the target to subtract 2d4 from their next saving throw?