Using AMP INSERTS on back of Carvin BX1200
 

Taken from the manual for the BX1200:

Link to image for rear of BX1200

21. AMP INSERT JACKS
The AMP INSERT jacks are TRS (Tip-Ring-Sleeve). The “TIP” is the SEND and the “RING” is the RETURN. Using a TRS (Tip-Ring-Sleeve) insert cable, the preamp signal can be sent out via the TIP to an external proces- sor and then returned to the internal power amp via the RING. If a standard (Tip-Sleeve) instrument cable is used, the signal can be routed from the preamp to an external power amp. These insert jacks break the signal to the internal power amplifier.
Use the MASTER (or AMP 1 & 2) controls on the front panel for SEND level adjustments.
When the X-OVER is OFF, both outputs receive the same full range signal. When the X-OVER is ON, AMP 1 receives the low frequency signals and AMP2 receives the high frequency signals.

I want to know if I could use the INSERTS in the same way you can use the RETURN of the effects loop, to bypass the preamp signal for that particular power amp and feed it the signal from another preamp source.. in this case a SansAmp BDDI? I'm looking for a way to use the two power amps in my BX1200 to create a 2 channel amp, each run into a separate cabinet. I will split my bass signal and run one side into the BDDI -> Power Amp 1 and the other side into the BX1200 input

if I use a standard Tip-Sleeve cable, can't I just insert if half-way to do this?

Why don't you just use the effects loop?

Because that would defeat the preamp of the BX1200.