Hamburking has a good point which is that storage
batteries should only be connected in parallel if they are on a constant maintenance
charge (float), or if they are normally off-charge, but are on a daily charge schedule.
Some heavy transport vehicles use paralleled starting batteries and this is OK as long as the truck is used on a daily basis.
The reasons include slightly differing effective terminal voltages, temperature differences (even quite small values). Terminal corrosion
films etc. can also start the loop discharge.
The sequence is usually a chase or spiral to a discharged state as first one then the other effectively supplies current
to the other -- and that is if both are nominally matched in age and specification.
If the group has one or more units which are not the same age, the problem is more pronounced.
Most Lithium-ion laptop
batteries are comprised of a S3/2P 6cell arrangement -- three pairs of tightly paralleled 3.7 V cells (18650) in series to increase the "Watt/hr" capacity of the nominal 11.1 package. These ususally self discharge one or more of the three sets even while the battery unit is removed from the computer. However, very high quality units self discharge only slowly due to close tolerances in their manufacture.
Bottom line is paralleling for starting OK, but separate unless being charged.