I believe Durhams is a separate investigation outside of Horowitz.
Parts of it certainly are, but there is probably some intersection on the investigation side.
Horowitz is investigate only, and DOJ only. DOJ includes FBI. In this case, Horowitz was given a narrow task, investigate the propritey of DOJ in obtaining FISA warrants against people on the Trump campaign - and maybe as narrow as looking at the Carter Page warrants.
Horowitz cannot bring indictments. He can refer facts to US Attorney, who have the power to indict.
Durham can do both investigation and indict, and his powers are not limited to the DOJ, but certainly includes the DOJ.
What I was talking about earlier was the order of public reveal. That order has been thought of as must be Horowitz first, then Durham. That Durham action depends on Horowitz report and referral.
To reiterate my point, that order is not for any determinative reason. Durham can "outrun" Horowitz, and certainly will for perps outside of DOJ, where Horowitz will have done little if any substantive work. Durham can "outrun" Horowitz on cases involving DOJ perps too.