Can most rear racks handle panniers and rack trunk(top pannier)? I am interested in getting a cheap rack, like the Topeak super tourist for my 2nd bike. Right now I only have panniers, but I would like to have the option of using a rack trunk someday. Do I need a rack with two sets of horizontal rails in order to accommodate panniers and a rack trunk?

I have the topeak super tourist (for disc brakes) and have no problem using it for both panniers (arkel shoppers/bug and the arkel trunk bag). Depending upon the trunk bag and how it attached, I could see a possibility for conflict though.

I've used a trunk + panniers together on a rear rack with just one set of side rails with no problems at all. The trunk I have attaches with velcro that loops under the platform. Other models may have issues.

You don't need a 2nd rail, but it sure makes things easier.

^^^^ This ^^^^

I got by with an ordinary rack for 18 months or so. There was a lot of cussin' and fussin' every time I wanted to use both the panniers and the trunk bag, but I could make it work,and lived with it for a long time.

The dual siderails rack I have now I just such a pleasure use that I don't care that it cost a lot more money, and I don't care that Topeak came out with a much, much, cheaper dual siderails rack shortly after I purchased mine.

Since you're already looking at a Supertourist, just go with the Supertourist DX and be done with it.