how to connect two grove 6-axis accelerometers in the same i2c bus?
Quoting the datasheet:


  The Slave ADdress (SAD) associated to the LSM6DS3 is 110101xb. The
  SDO/SA0 pin can be used to modify the less significant bit of the
  device address. If the SDO/SA0 pin is connected to the supply voltage,
  LSb is ‘1’ (address 1101011b); else if the SDO/SA0 pin is connected to
  ground, the LSb value is ‘0’ (address 1101010b). This solution permits
  to connect and address two different inertial modules to the same I2C
  bus.
  [Bolding mine]


So you select the slave address with the SD0/SA0 pin which is otherwise used for the SPI interface. Since you are not using SPI, you are free to tie it high or low to select the I2C address.