I'm designing a custom part that needs an active shock absorber and space is restricted. I most likely won't be able to use a stock absorber and was thinking, can two squarish tubes be made tight enough?
Assuming reasonable pressures, it should be workable.
However, keep in mind that the pressures will be higher at the corners and you'll run a higher risk of leakage / bursting. Cylindrical tubes distribute the pressure evenly across the bearing surface and don't have that issue.
You'll also potentially have difficulty with fabricating a gasket to seal the space between the square tubes.
Given the minimal difference in surface area between a cylindrical and square tube, and the ready prevalence of cylindrical tubes, it does beg the question as to why you feel a square tube is the more appropriate design choice.