Cell balancing of two batteries connected each at the output of parallel converters
So you have two batteries connected to each of the converters (lead-acid and LiPo) or does each converter shares a LiPo and a Lead-Acid in parallel?
Assuming you have two parallel LiPos at one converter stage and you think about balancing them, then I would start searching for information in the LiPo datasheet or some from the manufacturer producing these batteries because they probably have already a BMS (battery management system) integrated if they are rated as 12 V LiPos because a single LiPo cell has a nominal voltage of 3.7 V which means it will be a 4S cell (4 series).
Lithium batteries should always be balanced! If you find out that there is no BMS integrated in your LiPo batteries I would buy a suitable BMS and think about how to charge them safely with your bidirectional DC-DC converter.