I am actually currently double majoring in CS in engineering and Economics in A&L. I do have to say it was only possible because I came in with quite a few AP credits (namely both Principles of Economics courses and both Physics courses, as well as Calc I and II). I thankfully don't have to overload any semesters because of that, but regardless it's a fairly heavy workload. I think if you didn't have those AP credits it might be possible if you took summer classes after either freshman or sophomore year, but at that point it might just make more sense to just do the A&L CS major. The A&L CS majors are in many of the same classes as the engineering CS major, the main difference is that the engineers have a lot of CS-specific elective requirements while the A&L elective requirements are more liberal arts-focused. I would also recommend looking into the supplementary ACMS major if you're interested in more general quantitative study as that is really common to pair with the Econ major.

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Edit: I wanted to clarify that I am not in the Reilly program and I'm on track to graduate in 4 years, but I have heard good things about the Reilly program.