[willy]

Having boys and having them get into a scrap, even coming to blows is not that uncommon, serious but not uncommon.
That said when one pulls a weapon and stabs the other that is a different situation all together.
We do not have enough details on the boys, their age difference, size difference, whether one is living with a bullying threat. Any other personal relationship type things.
Drug use, alcohol abuse, even sexual abuse.
The assumption would be of course the one who is cut is the one who was not pulling a weapon., but the mere word assume means make an arse of you and me.
If you as the Father can not make these determinations and there are things underlying this confrontation you do not know about then for us to make a suggestion is rather hard.
It sounds like you may need to do one of two things immediately, first sit each down separately and do your best to determine what happened, insure your sons you will be there to help, you will be there protect, but you will also will call in the social services even police to investigate if you do not get the answers you need.
If you cannot get to the truth then you will have to make the call. Is the threat of further violence, maybe even escalated violence there? If so and you are unable to resolve it call for help. Don't wait until someone in your family is hurt or much worse.
Call for help.