Q See above question 1380 (Is the signature of the parties enough). Should the chosson better sign personally the kesuva in case it turns out that one of the witnesses are not kosher?
(Sometimes you want to honor someone, but are not sure of his private commitment to all mitzvos).

A. Tashbatz (Responsa 4: tur 3: 1), mentions that it was customary in his location that the groom would sign the kesuva, in case with the passage of time, when the kesuva came eventually to be collected, no witness could be found anymore to corroborate the signature of the original ones who may not be alive anymore or it is now forgotten who they were. However the signature of the groom would be easier to corroborate.
He rules that in such a case the former wife would collect even from encumbered properties (see above question). Nachalas Shiva (12: 73) writes that it was the tradition in Ashkenaz that the chosson would also sign the kesuva, but not in the same place where the witnesses signed, rather on the opposite side, so he would not be mistaken as a witness on his own kesuva.
This is the common practice today in many Sephardi congregations. Shevet Halevy (8: 285) recommends to have the chosson sign, even when many don’t follow that minhag today, since we usually follow the opinion of the Nachalas Shiva.
However, when the witnesses turned out to be disqualified by being related or not kosher, Tashbatz (ibid.) maintains that the signature of the groom will be of no help, since the document is considered as integrally falsified (mezuyaff mitocho – see Baba Basra 170). Mutzal Meish (1: 34) expresses a similar opinion. Nonetheless, Maharshach 3: 41) disagrees and maintains that the kesuva is kosher just by dint of the chossons signature.
Igrois Moishe (E.H.1: 100), mentions the chosson does not sign, as an outcome of being concerned he may sign in the same place as the witnesses.
Horav Shlomo Miller’s Shlit’a opinion is that in case of need, such as when there is reasonable concern about the validity of the witnesses (and they cannot be refused), one may ask the chosson to sign the kesuva also, being careful as mentioned above, to sign separate from the witness. The Rov added that such situation may be unlikely, since if we are uncertain of the validity of the witnesses, we would determine their status by placing them under chezkas kashrus or presumption of remaining kosher.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a