I want to cash in stocks I have with the united states and was told my wife and I need to file a w8ben form to avoid paying the tax, Do we sign the same form or do we both need to fill out the w8ben form separately?

QUOTE," . . . to file a w8ben form to avoid paying the tax, . . ."--- You are the true owner that is the beneficial owner.

"we both need to fill out the w8ben form seperately"---If your beneficial ownership be shared with your wife, then you need to file the forms in separation.

I want to cash in stocks I have with the the united states and was told my wife and I need to file a w8ben form to avoid paying the tax, Do we sign the same form or do we both need to fill out the w8ben form seperately?

Are you a foreign person? Did you live in U.S. in 2009 or 2010 and if yes, your visa?
If both you and your spouse are nonresident aliens, you need to file form W-8BEN. For nonresidents, income from stocks is not taxable.

File the W-8BENs separately.

Agreed; separate Forms W-8BEN is the correct course of action.