UART Software Serial - Syncing two different sensors with start and stop bytes
Your code bit 

SendCmd(0x00,0x5C);//Transmit 8 cycle 40khz pulse from address 0x00 - transmitter
delay(70);//time for SRF02 to measure the range
SendCmd(0x02,0x57); //Get Fake Range at Address 0x02 - receiver


will send a sound burst, delay 70 ms, then start listening for the burst, which by then will have traveled around 24 meters and probably be no longer audible.

You should instead start the fake range listener immediately after sending the burst.  The general idea for one approach is as follows:


Record time (eg, t1=micros();)
Send burst (eg, SendCmd(0,0x5C);)
Record time (eg, t2=micros();)
Listen for burst (eg, SendCmd(2,0x58); // Get microseconds)
Wait 70 ms, then read microseconds result t3
Compute corrected total microseconds, tm=t3+t2-t1
Convert microseconds to cm


Note use of command 0x58 instead of 0x57 to get microseconds instead of cm.  If you used 0x57, instead of adding t2-t1 to t3, you would add t2-t1 microseconds-converted-to-cm to a cm reading.

The reason for the computation tm=t3+t2-t1 is that the second command is issued about t2-t1 microseconds later than the first command, as explained in the next paragraph.  

There is an unknown delay d1 between when t1 is recorded and when the first SRF02 acts; and there is an unknown delay d2 between when t2 is recorded and when the second SRF02 begins listening.  The correct amount to add to t3 is t2+d2-(t1+d1), which is t2-t1 if d1 equals d2, a more-or-less plausible assumption.  If your experiments reveal a constant error, add a small constant offset to compensate.

I doubt you should have the delayMicroseconds(100); statements in SendCmd(), and would recommend getting the code to work ok, then see if it still works ok when you take them out.  The delay(10); is unnecessary if you rewrite your 2-character-wait correctly (ie as a while rather than an if), and the delay(70); is unnecessary if you start conversion with command 0x5A,  “Fake Ranging Mode - Result in centimeters, automatically Tx range back to controller as soon as ranging is complete” and then properly wait for two characters to arrive, instead of a blind wait followed by a test if they've arrrived.