It seems that when you talk to the golden retriever, options will be displayed.
On a side note, can't you see the sub-weapons you have in your possession from the menu items?
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Seems like I overlooked one. Thanks fixed that!
They only can be seen by going into the sub weapon tab to equip one. I mainy did that because the equipment screen was getting very clusterd with all the different types inside. I could make them appear in there, just don't know if that would be a good call.
There are already enough entries that more cease matter in complexity. The lack of access to the sub-weapons in that menu is IMO worse than the extra entry. Increased division on a higher level makes it also easier to look for the stuff you care about, instead of having to ignore dozens of actual item entries.