Rava says that even if siman is d'oraysa, it would still not measure up to eidim, therefore we would return the object to the one who provides the eidim (rashi). The Maharshal points out that the gemara is only able to tell us that eidim trump siman after it established that siman is d'oraysa, which implies that this would not have been the halacha in a case of siman muvhak against eidim. Rather, we would leave it until Eliyahu comes, because siman muvhak is just as good as eidim. The Shach (267:7) disagrees and holds that eidim is better than a siman muvhak. The shach understands that there is no more of a chiddush in saying that eidim is better than siman muvhak than saying that eidim is better that a standard siman which is d'oraysa. This would imply that if siman is d'oraysa and we have siman against siman muvhak, we would not return it to the siman muvhak, rather we would leave it. Pischei Teshuva quotes R' akiva eiger who is not sure about what to do when siman is up against siman muvhak. The question is that the gemara makes it clear that we always follow the better siman. If one gives the dimensions of length and width and the other only knows the total, we return it to the first. Based on this it should seem obvious that we return it to the one with the siman muvhak, because even if both are d'oraysa that is certainly the better of the two simanim?
To answer for the Shach and r' akiva eiger (who at least considers this an uncertainty), it seems that really we don't follow the better siman. Meaning, if siman is d'oraysa and both provide a siman it doesn't matter that one is slightly more specific than the other, and therefore we are required to hold the object until Eliyahu comes. Therefore in the case of siman muvhak against a standard siman which is d'oraysa we also don't return it to either one. However, the case where one gives the specific dimensions of length and width and the other gives the total, since the siman of the first completely includes the second and adds to it something more specific, it is as if the second has no siman at all and we return it to the first. But if both simanim are completely different even if the second is better we don't return it.