BQ24170 li-pol charger
You are right. According to manufacturers website the BQ24170 chip has a fixed 4.2 charging voltage. You cannot adjust it.
This site http://www.ti.com/product/bq24170 has a comparison table for these chargers. 
BQ24171 and BQ24172 are adjustable.


  (bq24172) Charge voltage analog feedback adjustment. Connect the output of a 
  resistor divider powered from the battery terminals to VFB to AGND. Output voltage > is regulated to 2.1 V on FB pin during constant-voltage mode.


So make a divider to pin 14 ( FB ) as suggested : 


  The bq24172 uses external resistor divider for voltage feedback and regulate to >internal 2.1-V voltage reference on FB pin. Use the following equation for the >regulation voltage for bq24172:
  V =2.1V
  where
  • R2 is connected from FB to the battery.
  • R1 is connected from FB to GND.


For 2x 3.7 = 7.4V   R2/R1 = 2,52380952380952.

Using bq24170 you can adjust charge voltage for multiple cells but in 4.2 increments. If you want 3.7 / cell you have to use adjusted version.