Disk Utility will not let me resize Mac OS Extended (Journaled) partitions

Only the El Capitan partition has the space to be resized. You need to remove a portion of the El Capitan partition anyway. It is hard to say whether the freed space could be merged into the Mojave partition because it depends on which partition is the first one at the beginning of the drive and Disk Utility doesn't make it easy to determine the partition layouts in that regard. You also cannot be booted to the partition you are resizing. Probably best to try it from Recovery Mode or Internet Recovery Mode.

You may need to erase the whole SSD and start completely over by reinstalling macOS & restoring from a backup. Sometimes it just is not possible to resize partitions.

I hope you have a good backup of both volumes just in case something goes wrong since modifying partitions is a major & risky endeavor. This is why I always discourage anyone from creating multiple partitions on any drive because almost always at least one partition will not have enough space. It is much better to install a second OS on an external USB3 SSD or a second internal drive if the device supports it.

I believe you will only be able to resize the area to be smaller, on the El Capitan partition. The hashed area on Disk Utility typically indicates an area that can't be changed.