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In general, yes, adding another box in active-active will increase the number of users supported.
It will not be double (maybe 1.5x, but it depends on a number of factors), and it also introduces limitations as documented in the SK @RamGuy239 pointed to.

In general, I advise against ClusterXL active/active and instead to purchase something that is able to handle future expected load (3-5 years depending on the company) versus buying precisely what will solve the need today.
This accounts for both future user growth but also additional overhead for additional security functions down the road.
If you need something that can quickly scale up by adding more appliances, look at a Maestro solution, which will give you nearly linear performance gains by simply adding another appliance.

Of course, all of that is general advice based on the very general information you’ve provided so far.
If you want specific advise, we need to know in more detail what you actually have and are using today.
It might also be a better conversation to have with your Check Point SE or partner.