Yes, you must send in both forms or the IRS will disallow your deduction for that donation.
Here is an excerpt from the IRS forms -
"Caution: You must attach Copy B of Form 1098-C to
your income tax return in order to take a deduction for
the contribution of a qualified vehicle with a claimed
value of more than $500. (If you e-file your return, you
must (a) attach Copy B of Form 1098-C to Form 8453
and mail the forms to the IRS"
https://www.irs.gov/pub/irs-pdf/f1098c.pdf
