Very low, unless...
Two virgins, if they have had no sexual contact at all, have a very low chance of carrying stds.
However, there are 2 reasons not to count one this.
1) virginity is a subjective state (some "virgins" have had anal and/or oral sex
2) some stds can be transmitted via blood or mucosal membranes as well, so virginity is not a guarantee of being disease-free.