Trim is difficult because If i understand what you want the parts you want to remove are on the inside and difficult to select.
Much easier to use Split command. In the enclosed file first select the green object and run split and select the pink as cutter. Then with all the split parts selected deselect the outer part of the green and hit Delete to get rid of the parts you don’t want. Then select the Pink and use the green as cutter. Then again deselect the outer part of the split result and hit Delete. Now you have the two parts that will join to a solid. water shootX.3dm (316.6 KB)
If you want to use Trim you can set up a clipping plane thru both parts and then you can select both parts and run Trim command and click on the internal parts that you want to remove. This leaves with the same result as described above.