Is it ok to use digital potentiometers in parallel?
While in principle you get  higher resolution in practice you do not as the code words are in simple case  redundant and in complex case you root square sum the differential non linearity and overall error. Because the resistance is in denominator this error has a large effect on resistance error.

For precision device a dedicated 5K pot would be best, but even the 10K pot at half resolution will likely perform better than twin pots . 

For twin pots you would need to perform and maintain a calibration set that gives you valid combinations of codes to have linear step of the total real resistance, otherwise you are liable to have very non linear  lsb step response relative to single pot if you use uncalibrated approach of setting the same code on both for half resistance. Or any scheme that does not require maintaining calibration values and picking the best set  among nominally redundant control codes.

E.g you want 2.5K, should you pick 5K and 5K or 5K1 and 4K9 , which would give you the most linear step from 2.5K+1LSB, which would give you the most linear step to 2.5K-1LSB? What if that is different from the codes that give you precisely 2.5K not 2.5K+integral error?