You can grab power from the 3.3v lines on the Pi’s GPIO header. I had a circuit board made for mine. The exact wiring is going to depend on the ADXL345 boards you use. There are a couple of different ones out there with slightly different pins.
I wired mine to SPI 5 & 6 on the Pi. So that’s pin 39 for ground and 17 for 3.3v, shared to both ADXL boards. (There are two choices for 3.3 and several for ground. Any will work.) Then 8, 10, 32, & 33 for one ADXL and 12, 35, 38, and 40 for the other. Match up the signal names from the Pi header for the SPI & CS lines to the pinout on your ADXL345 boards.