Hi Minh,
Welcome to the forum!!
A voltage converter will have some maximum rated power - dependent on the difference in voltage. For the Pololu regulators there are plots in the product pages.
You definitely can connect them in parallel. In series the efficiency will compound rather than add. Using the raw DC voltage for both regulators (and making sure they can handle the max voltage from the battery, as the battery discharges the voltage will drop).
Also I can’t reiterate this enough LiPo cells store a lot of energy. Make sure you have to correct charge profile and don’t over discharge them.
