Single or two sided finishing?
When working with sheet metal, one of the more common questions is around this very topic. First there will be outliers with any topic or generalization. A little bit of how it works will go a long way to help understand if that is the correct solution for your need.
Single side finishing:
The part will typically lay on a feed belt that is pulled into a machine. The feed belt and working table provide rigid support for the part as well as its movement through the machine. The abrasive tool will come down from above the part and work the surface or edges as the feed belt moves it through. The working table that the feed belt is laying on acts as the support to keep the part in position and not let it push away from the abrasives. This is the default standard situation in most finishing machines. You can apply a fair amount of pressure to the part as the working table will support it. You will have access to a large variety of tools and upkeep is straight forward.
Dual side finishing:
The part will typically lay on rollers but there are many versions are out there. The rollers will be powered and are what pull the part through the machine. As the parts move through the machine they move roller to roller until they exit. You do not have the working table to to act as the rigid support for the part in most cases so you cant get as aggressive with the pressure. As you try to apply more pressure the part can just deflect as there is no solid table supporting it. As the part is supported by rollers you need to be aware of the part size requirements. The manufacturer should be able to tell you how short of a part you can safely run. With the rollers acting as the feed and support if the part is too short it will act like a tiny boat on a rough ocean and bob up and down over the rollers or fall down between them.
Single side finishing will offer the most aggressive work and allows you to use a heavy hand in most cases. The single side machine will be lower cost and less complicated in general. Dust collection tends to be easier and more straight forward. You will have more variety in machines you can pick from to get the best fit and cost.
Two side finishing will be less aggressive due to the lack of the support from a working table. You will likely not get heavy rounding from the tools for the same reason. It will be far more complicated and carry a much higher cost as well. You tend to be more limited on the tools available so some of the abrasives you want to run may not be possible.
Who is two sided finishing for? Material that is relatively thin with good cut quality is the typical use case. Brushing from the top and bottom is enough to just break the edge and do a little work to the surface. There are some real monsters out there for working with plate. The plate machines need to be able to support the weight of the parts and have the force to pull them through the machine. These tend to have feed extensions and roller tables to ease the loading and unloading. They are also somewhat rare due to the size and cost of the machines. They have to be monsters due to the forces involved from the weight of the material to the horsepower needed to pull them through the machine while being pressed down on by the tools. You have to pay attention to the details with these types of machines and be clear on your needs. If you are going this route either do it completely or not at all, low end machines that claim to be for plate with a low buy in are reason to worry. Everything in the plate machines needs to be beefed up compared to a standard machine so the cost will be much more not less.
In most cases even if you need to do both sides it ends up cheaper and with fewer headaches to just use a return conveyor or put two single sided machines in line. The ability to use more standard tools and be more aggressive tends to compound with the lower machine cost to make the decision. If you have zero space or need to put a machine into an existing line where both sides need done there is a case for the two sided machine. For the vast majority dual sided machine are not worth buy in even if they sound great on paper.