yes, u can do that, since your BG is ground referenced.
But u have to be aware of situations where the output from your 2.5V system(the buffered bandgap)exceeding the common mode of ur 1.8V bandgap referred current buffer. therefore, u have to check, in transient, what happened when supply glitches high(simulate both high and slow slew rate), during power up, again, high and low slew rate, across Process and temperature. Normally not a problem, but good to do as a sanity check.
You also have to watch for one supply coming up before another supply. u should handle this kind of situation by having some kind of undervoltage lockout circuitry.