If we have two RF signals of 900Mhz and 1800Mhz both of equal Bandwidth 20Mhz which one has higher data rate?
The value 900MHz and 1800MHz refer only to the carrier frequency. A pure perfect sine wave doesn't transmit / carry any information. Only when some parameter of the carrier changes, is data transmitted. This process is called modulation. The parameter could be something like frequency, phase, amplitude etc. More frequently you change the parameter, more data gets transmitted. The frequency with which you modulate the chosen parameter determines the bandwidth. So a signal with higher bandwidth transmits data at a higher rate. Two signals with same bandwidth and modulation scheme at two different carrier frequencies transport data at the same rate.
If the signal to noise ratio is identical for both systems and they are both presumed to use the same modulation method, the data rate (C) is proportional to B, bandwidth and nothing to do with the carrier frequency.
If, as you say, everything about the two systems is the same (modulation, etc.), and the only difference is in the carrier frequency (one at 900 MHz, the other at 1800 MHz), then the data rate would be the same, ideally.
In practice, if there's more interference in one band than other, if the performance of RF components varies from one band to the other, etc., you may observe differences. But I don't think that's what you asking about in this question? So, then the answer would be that they are the same.