i know three methods for producing two quadrature sine wave :
1.produce a frequency two times the required freq.,then use two cross coupled D-Flip Flop to devide freq. by two,so far u have two exactly quadrature output,but yhe outputs are squre wave simply use active filters to make outputs sine wave.
2.produce a sine waveform at desired freq,then apply it to two simple RC filter,both have the same R and C,one of them in lowpass config. and the other in highpass config.in this way u have two outputs,which have a 90 deg phase shift relative to each other,accuracy of this circuit highly depends on accuracy of R an C,thus for high presision applications it is necessary to use 1% tolerance resistors and capacitors.
3.use DDS,or simply write a asm. code for 8051 with a one cycle of sine wave look up table and,read the look up table with a constant freq.(timer int.) and each time two number with correct index shift,and write the values to two ports of 8051,by using two low cost DAC at ports u can achieve very high precission waveforms with a simple filter.
best!