Hi,
I downloaded some music sheets and I noticed that on some on them there are two bass clefs with one treble clef? How to play this?
Thanks

No clue unless you can post a picture.

probably Cello/bass pieces with piano accompaniement.

I could not post a picture but I can post the link to the pdf file:
Double bass clef

The second one is for the (electric) bass. Given that this really isn't a piano transcription, either stick with the top two lines or play what you like. Those long synth chords aren't going to work too well on a piano. I would be tempted to pulse them at half note intervals quietly.

The brace connecting the two staves at the left, indicates that these are for piano. The bottom staff is for some other instrument that reads bass clef.

That's very strange. Yes, the bracket indicates that the top 2 are together with another instrument playing the bottom. However, the top staff has only the melody. So if you're singing along, it'll sound strange. I'd just use the chords marked as a guide. And if you're playing it as a piano solo and play the top 2 only, you're going to be missing some tonality by not hearing the bass note. Either way I think it'll sound a little off. Maybe look for another version. One that doesn't have Sinead O'Connor's name associated with it. Prince's version is SO much better.