LTC3350 and LTC3351

What exactly the difference between the LTC3350 and LTC 3351?

LTC3350 is Buck charger and Boost converter right ? Does that mean If the my supply is 18V Output Voltage will always be less than 18V right ?

And about Boost converter it'll only boost the SuperCap stack Voltage not the input supply right?(like If I use supercaps of 2.7 and stack them 4 in series (10.8) and Output Voltage can be 12V?