Using two modulators is more complex then it needs to be. The simple way is to modulate both signals on a single carrier and then differentiate the signals by using two filters on the demodulated output. The demodulated output will consist of the 100Hz and 1000Hz signals. A 100Hz low pass filter will pass the 100Hz signal and suppress the 1000Hz signal. Similarly a 1000Hz high pass filter will pass the 1000Hz signal and suppress the 100Hz signal.
The amount of suppression is determined by the number of filter poles in each filter. A simple, single-pole RC filter will have a suppression of 20dB/decade (a factor of 10) which is what you have here since the signals are a decade apart.
If you need more suppression you would use a higher order filter. Each increase in filter order improves the suppression by a factor of 10 (20db). An op amp two or three pole active filter would work well. A good design tool for designing active filters is from TI:
Active Filter Design Application - FILTERPRO - TI Software Folder.
EDIT: The above won't work, of course, if the two modulating signals are near each other in frequency or overlap in frequency, such as two audio signals.