Witness on sale deed

There's nothing wrong in they sign the sale deed as a witness.

But why do you insist on them alone 

I have to give name of the person and Id of that person acting as witness before registration. In any case if that person couldn't come there may be problem. So I just want to be safe.. my parents would be there with me at the time of registration. So I just wanted to know if they sign as witness.. is there any problem with that? One more option my father sign as a witness and for another witness i should ask seller?? Please suggest.

Both the options are valid and legal since the witness(es) has to be the person(s) competent to contract in terms of Section 11 of Contract Act, 1872.

Any person ( relative,friend,neighbour or known person ) may be witness of document but he  should be adult and sound mind.