Referee wants me to do more experiments, but I already submitted these results to another journal
I think the normal thing to do is to forward-reference the experimental results. 

That is, you can briefly outline the experimental results as per the current referee's desire for more information, and reference it as "paper submitted to xyz journal, 2/9/2017."  Down the road, if the experimental paper turns out to be accepted before the theory paper, all the better and you can actually reference it as such.

As a caveat, it may also be true that either paper is too thin for separate publication, and the two should be combined into one. This sounds like what the referee is getting at. From your point of view, having one strong paper may be more desirable than two weak ones. 

Finally, most often I have seen two papers of this type published back-to-back in the same journal.  But even if this is appropriate here it sounds like it may be a bit late to go this route, unless you were to withdraw one and resubmit the pair. (BTW, If you do this, an explanation to all the editors involved should definitely be included in your correspondence.)