In the File ConfiguredFeatureModule.java the paths for the double arch structures point to the arch folder instead of the double_arch folder.
eg. swamp/arch/swamp_double_arch_0 should be swamp/double_arch/swamp_double_arch_0.

Log file dumb a error and the double arch will not spawn.
eg. Failed to create invalid feature yungsextras:swamp/arch/swamp_double_arch_3

Edit: the File swamp/arch/swamp_arch_11 is not existent but is in the file mentioned.

Good catch, thanks!