Hi,
the term DC-DC charger usually mean a device hooked up to ensure that an alternator in a car will correctly maintain an auxiliary battery in the back of the car or a van battery. It overcomes the low voltage of a smart alternator and the voltage drop on long runs of cable from the car back to the battery.
The device used to connect between solar panels and batteries is called a solar regulator or controller. There is also the term BC-DC charger which is similar but subtely different. Since I haven't a need for one, I have never bothered to understand the differences.
Also, many devices provide more than one feature so it becomes confusing. Some also provide charging from 240V as well.
These are more commonly grouped together and called Battery Management Systems (BMS).
I have no experience with what I call DC-DC chargers. But since your post seems related to batteries and solar panels, the following is some information on solar regulators or solar controllers.
Regardless of whether you wire the panels in series or parallel the panels will deliver the same wattage.
If you wire the panels in parallel, you will get same voltage as a single panel but double the current.
If you put the panel in series you will get the same current as a single panel but double the voltage. You can use either a PWM or a MPPT solar controller
In parallel, since you hav twice the current, the wiring down to the controller will have to take twice the current and thus will need to be much heavier.
In series, you HAVE to have a MPPT solar controller to convert the higher voltage down to the battery voltage.
With multiple panels you have to consider the issue where one panel is in the sun whilst the other is in the shade.
With parallel panels, Diodes are placed in series with each panel to prevent a panel in the shade from becoming a load on the panel in the sun. These are called Blocking diodes.
With panels in series, a diode is wired across each panel so that it is bypassed when the panel is in the shade. Naturally enough, they are called Bypass diodes.
"In the shade" can also mean, the panel is on the roof and covered with a big splash of animal poo.
MPPT controllers are more efficient than PWM controllers but they are more expensive.
For the size and cost of the panels you are considering, it would be worth investing in an MPPT controller.
You will need to read the specifications for any controller to ensure it will handle the maximum current and maximum voltage that the panels can generate. In good sunlight and cool temperatures, a panel can easily produce 20% more current than the Maximum Power current of the panel. (Ipm).
Further, the solar controller will have to support your lithium batteries.
take care
Mike