In order to join them together, you'll need to cut the rubber into 100mm strips and wrap it around the 25mm pipe. You'll also need to wrap some on the 50mm pipe. Hold them in place using cable ties, make sure to get one that is rated for external use. Place the 25mm pipe over the 50mm pipe where you want to position it. Clamp both pipes together using the "U" bolt. Please note your goal is to clamp bother pipes together using the rubber matting as fillers. If you feel you need to put more on don't hesitate to add more. Keep in mind that excessive force will crush not only the 50mm pipe but the 25mm pipe as well, your goal is to have it firm and steady but not overly tight.
If you were to drill the pipes together, I suggest using the rubber matting as a spacer in conjunction with Zenith M10 x 100mm Stainless Steel Cup Head Bolt And Nut. This configuration would be straightforward and would need less adjusting compared to clamping. But it is still subject to crushing so please do not overtighten the nut and bolt. I've placed a basic sketch below, if you have any questions about the configuration, please don't hesitate to post them.
If you need further assistance, please let us know.