There is not. Square cannot share inventory between two locations on the same account, so sharing between 2 accounts isn't an option.
My best advice would be to either:
(1) Do your best to separate your items and unfortunately have to create two separate transactions if they want items from each of you,
or (2) Use one account and import the inventory of the other into the one. For correct records, you would probably want to do a purchase order and a payment to the other account for the items that belong to the other person. This is going to be a little tedious and time consuming.
I guess it depends on how many people will shop from both of you at the same time. If it's a lot, then setting up one account would be best, if you don't think it will be more than a few, I'd keep it separate. You may even consider setting up a third square account that includes both inventories if you think this will happen frequently. Although, this will create more accounting work.
If you do use one account, don't forget to deduct the square fees when paying out the other party.
