Can I add a second 100A service panel alongside the existing one on the same feeder lines?
Not necessary.
All you need to do is get your 200A service, feeding a new service panel that is a 200A “main panel” with main breaker.  Then inside this panel, stick a 100A breaker,  then run 1 AWG aluminum wire from there to the old 100A panel.
The old panel must be re-configured as a subpanel, which requires removing all ground wires from the neutral bar, putting them on an accessory ground bar, and removing the neutral-ground bond.