Can charge pumps be combined?
If it were my project, I would lay out the 1st version of the board with individual charge pumps.  Then I would populate just one and connect the remaining controllers with a blue wire (as you're proposing).  Test worst case.  If it doesn't pass the test, you'll have 7 more charge pumps to come back to.

Judging from the datasheet and comments above, it should be possible to share output of one charge pump across several motor controllers.  If I were to do that, I would hook up Vboot pins together and fully populate one charge pump (of course).

Will the charge pump generate enough current for 8x motor controllers?  It depends on the frequency and the gate capacitance of the MOSFETs that controllers are driving.

On the other hand, this is not how ST (who designed the chip) intended for the charge pump to be used.  You would save 14 capacitors and 7 dual diodes.  You decide whether it's worth the engineering risk.