Can linear guide rails be joined?
The answer is yes; and it's a normal thing to do.

https://www.igus.com/info/linear-guides-butting-rails-together



Basically you should only do it with rails from the same manufacturer and you might have to file the mating surfaces, but butting them is altogether trivial. This method works with Drylin and UHMW-PE bearing surfaces as recirculating balls could get caught in the small gap that is left behind. This would cause the carriage to "jump" as it passes over the joint.