splice not working correctly

Hello,

I'd like to use split to mix two slightly different tracks for AB
testing purposes. I'd like to have 0..5 second from track A and 5..10
seconds from track B, etc. 10..15 track A, 15..20 track B.

I'd be very important to make sure that it doesn't change the signal
in any way, as in if I run it with identical tracks, I'd get back the
original track.

My problem is that it's not what is happening. For validation I'm
using a big excess value = 1 sec + -q for equal cross fade.

So far here are my basic CLI arguments:
sox A.wav tmp/A_0.wav trim 0 6
sox A.wav tmp/A_5.wav trim 4 7
sox tmp/A_0.wav tmp/A_5.wav out.wav splice -q 5,1

Can you correct me in how to use splice correctly? The way I'm using
it I get a bit of an echo / duplication at the end of the first file,
just before the mixing point.

Zsolt