That depends on the monitors. If they're active monitors and the sound people allow it, you could probably do that. You may be better off going direct to the house mixer and getting them in your monitor mix along with other things you need to hear.
If you are doing stereo stuff with your keyboards it may be worthwhile having two monitors on separate mixes. Summing a stereo keyboard feed to one speaker can alter its sound. If you only get one monitor and mix, maybe have them put just the left channel in it, and put the right channel in the monitor mix of the musician next to you.
This is a good example of why you should have your own stereo keyboard amp with direct outputs. You would have proper control of your own sound for your own monitoring purposes, and the house system could take care of getting your sound to the rest of stage and the audience.