Not knowing what you are looking to do, we can only assume things here. I assume the blank is a CF clear resin coated blank. I assume the tube that is installed is a 10mm tube. I assume you own a bandsaw with a fine tooth blade. I assume you need the full length of the blank for 2 sections of a pen. As you see too many variables untold.
I will give you some ways to cut. If you have a bandsaw with a fine tooth blade that would be the quickest way. I would make a "L" shaped sled to support both ends of the blank when cut. This will keep the blade from grabbing the blanks and possibly deforming it. You can also do this on the tablesaw with a fine tooth carbide blade. Again a sled should be used.
Another way would be to do it on the lathe and use the skew or a parting tool. This would require you be able to chuck the blank squarely. You will loose more material in the cut this way though and may not be what you can afford.
If you are still worried about crushing the tube which will not happen because it is supported and adhered to the resin. But if still worried you could always slip a dowel rod just small enoughinside the tube to support it. Don't have a dowel you say, just make one. That is what a lathe will do. Take ablock of wood and turn down to inside tube measurement.
