Adafruit's rectangular 9DOF device, unlike their round 9DOF, brings several more signals out to pads. When using I²C with the rectangular 9DOF board, as I understand it signal pin SA0_G can be set low or high to modify the device address:
The SDO/SA0 pins (SDO_XM/SA0_XM or SDO_G/SA0_G) can be used to modify the least significant bit of the device address. If the SA0 pin is connected to the voltage supply, LSb is 1 (ex. address 0011101b) else if SA0 pad is connected to ground, the LSb value is 0 (ex. address 0011110b).
Controlling that signal should let you put multiple 9DOF's on one system. (See my answer to question 13830 for some additional comments on setting an I²C address.) Also see tables 15 and 16 on page 33 in the spec sheet (DM00087365.pdf) regarding addresses.
If you insist on using round 9DOFs for this application, you would need to find the trace from pin 22¤ of the LSM9DS0 to Vcc on one of your 9DOF boards; working under a microscope with miniature tools, cut the trace and rewire the pin end of the trace to Ground. This will change that board from using addresses 0x1D and 0x6B to 0x1E and 0x6C, if I correctly understand the spec sheet.
It probably would be a good idea to get one of the rectangular 9DOF's so that you could experiment with changing its addresses, before modifying one of the round ones.
¤ Looking at the top of the chip, pin 24 is just clockwise from the depressed round dot marking pin 1; pin 22 is 1.75 mm from the corner. See Figure 21, LGA 4x4x1 mm 24-lead outline, in the spec sheet. That figure shows the underside of the chip.