The lights will work wired across two batteries but it is definitely not recommended for lithium batteries. The pack will quickly become unbalanced and the balance function on the BMS is only designed to correct small balance errors. it will not be able to bring the pack back into balance in one charge cycle. On the next discharge cycle, the pack will become even further out of balance.
Since the BMS is designed to protect the pack, it will shut down power if any one cell becomes discharged, therefore, your total pack capacity is only as good as the charge held in the least charged cell. Your aH pack rating will diminish with every charge/discharge cycle. Eventually, you will get to the point where the 2 modules (4 cells) become totally discharged while the other 5 modules (10 cells) remain almost fully charged.
You will think that your pack is no longer holding charge, when in fact it is the BMS that is protecting the cells from over charge and over discharge. The pack can be restored by charging the 4 discharged cells back to the same level as the 10 charged cells.
A 48v to 12v converter is a MUST with a lithium pack. You can use a second battery but a converter is much more convenient.
Cheers
Pat.