Ok, I’m not going to get into specifics for your situation because you did not provide information on your current solar system components.
1. Solar panels can be wired in series or in parallel. First, the two panels should be identical or really, really close to identical in their specs. Mixing a 140 watt panel with a 300 watt panel is not a good plan. Generally, but not always, you would wire them in parallel to increase the amperage output with out increasing the voltage output.
2. You don’t just wire solar panels to a battery. First the wiring, using MC4 connectors, needs to be properly sized for your panels power output and the distance from the panels to the solar controller.
3. You need a resettable fuse in the positive wire running from the solar panel(s) to the solar controller. The fuse should be rated at about 125% of open circuit amperage for each panel. Again, it makes a difference if your panels are wired in series or in parallel.
4. You need a solar controller that is sized to handle the amount of volts and amps the solar panels and wiring are delivering to it. I believe it may be possible to use two panels independently for the Morningstar Duo but I almost know nothing about that controller so I will let someone else chime in. But if I had a Morningstar Sunsaver PWM solar controller I would replace it with a more efficient MPPT controller like those from Victron Energy.
5. Then you need a second fuse between the solar controller and house battery you are connecting the solar system to. That fuse needs to be sized to match the power output from the controller. The wiring needs to be sized correctly as well.
As to a lithium battery being a direct plug and play replacement for AGM or flooded batteries the answer is NO! Lots of things need to be changed (battery charger settings, solar controller settings and possibly a new alternator, etc.). Not my area so I’ll let someone else explain what needs to be done.
For those solar experts out there, please correct me if I’ve misstated any of the technical aspects of doing this solar panel upgrade!