I have conducted a test, as follows:

I mixed a small batch of just part 1 of the Blix, 1+4 with water, no part 2 added. Temperature at 38C.

I placed into the solution a black and white 35mm negative (Neopan Across 100), previously developed and fixed normally (a poor shot I didn't care to keep, of course).

After 4 minutes, no change occurred to the negative. Image was still fully intact.

After 8 minutes, no change occurred to the negative. Image still whole.

I added the appropriate amount of part 2 to the solution. This would mean that the solution was slightly dilute, due to 1 part too many of water, but I figured this would not be critical to the test.

After cleaning the still pristine negative under running water, placed it in the "fortified" solution and restarted the timer.

After 2 minutes, the image was nearly gone.

After 4 minutes, the image was completely bleached (and, I presume, fixed, though it had been fixed already...).

Conclusion: Tetenal Blix cannot be split. Part one diluted to a normal working solution is completely inactive, at least in any reasonable timeframe.