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?
Yes it does.
The rules for combining magical effects state:
The effects of different spells add together while the durations of those spells overlap.
This rule goes on to impose stacking restrictions on separate castings of the same spell, but explicitly permits the effects of different spells to add together.
So if a creature finds itself under the effects of both bane and mind sliver, then they would subtract 2d4 from the next saving throw they make (before the end of the next turn of the caster of mind sliver).