If you want to run two or three different promotions at once, you can track all of them. Just be careful about it. The problem with traditional mailing list advertising is that while you may know where you're spending your money, you may no clue where your customers are coming from (unless of course you're running only one ad).
The advertising is too general and isn't keyed so you can tell what generated each response as it occurred. Which ad did they see? Did they watch the TV commercial, or find you the newspaper, or in the Yellow Pages? Was it a radio ad that mailing list worked, or that postcard you mailed? When that happens, there's no way to figure out the ROI. Oh, in the abstract you may be able to calculate some loose numbers, but there's no way to specifically account for each dollar of advertising.
Did it come from upselling, down-selling, or cross-selling? You have to constantly look at those options and know at least that much, because mailing list not everybody buys. It will never ever happen. Some of each new group of prospects coming into your business aren't going to respond positively to your offer; you need to know that, so you can try an alternate method to get them to respond.