First, see this question for some extra context. As a general statement, you can use instrument training to count towards commercial training, but only if you can demonstrate that it met the commercial requirements.
If you look at the commercial XC requirements, you can see it explicitly requires that the training has to be to commercial standards:
(3) 20 hours of training on the areas of operation listed in
§61.127(b)(1) of this part that includes at least
[...]
(iii) One 2-hour cross country flight in a single engine airplane in
daytime conditions that consists of a total straight-line distance of
more than 100 nautical miles from the original point of departure;
So what does that mean for you? Well, if you can prove that your IR XC training was to commercial standards then you can double dip it. Perhaps you're at a flight school that deliberately plans and documents things that way. Or your IR instructor is willing to endorse your logbook with the remark that the IR XC was conducted to commercial standards.
On the other hand, if you did your instrument rating ten years ago and only started working on your commercial recently, it would probably be tough to "provide evidence" (as the FAA puts it) that your IR XC was indeed to commercial standards.
You could ask the DPE who'll do your checkride what they're looking for, although that draws attention to the question and you might not know which DPE you'll use. So all in all, if there's any doubt about whether your IR XC was to commercial standards, the simplest thing is simply to go fly a new, commercial XC with your instructor.
