If I understand you correctly, you want the drive_L variable to be used for both PWM16 and PWM14, and the drive_R to be used for PWM13 and PWM15.
One way you could do this is to place each of those two variables (drive_L and drive_R) into the positions for each of the two PWM outputs in the SEROUT command.
Code:
' Serout USERCPU, OUTBAUD, [255,255,(PWM1),relayA,(PWM2),relayB,(PWM3),(PWM4),(PWM5),(PWM6),(PWM7),(PWM8),(PWM9),(PWM10),(PWM11),(PWM12),(PWM13),(PWM14),(PWM15),(PWM16),(OSC)]
SEROUT USERCPU, OUTBAUD, [255,255,127,relayA,127,relayB,127,127,127,127,127,127,127,127,127,127,drive_R,drive_L,drive_R,drive_L]
The first line is the example SEROUT that IFI provides in the default 2003 EduRC code, so what I did was place the drive_L and drive_R variables in the string of output bytes where the system will be expecting PWM[13-16]. I set all other PWM values to 127, but you can replace these with whatever you'd like.
I hope this helps. If I misunderstood the question, I blame the fact that I just woke up...