Both

1a. Both him and Sita have rejected their allegations.
1b.Both he and Sita have rejected their allegations.
2a. Their allegations are rejected by both him and Sita.
2b. Their allegations are rejected by both he and Sita.
Please correct the above sentences.
Thanks.

Do not use 2b.

Beeesneees,
What about 1a?
As subject can we use ‘him’? Please clarify.
Thanks.

I don’t think you can in the active voice. It should be in the subjective case as in 1b.

It is used and widely accepted now informally.

The poster is a discerning user of English who seems to learn and understand the difference.

Which ‘poster’ would that be?