Phasing is only a problem or concern when you have 2 circuits running in parallel at the same time. If the 2 circuits have opposing phase, then there can be cancellation of frequencies when they combine. Since your 2 parallel circuits will never be operating simultaneously this shouldn't be a problem for you.
But then there is the issue of absolute phase of an amp (i.e. a positive input signal from the guitar results in a positive output signal at the speaker), but that can really get complicated and is probably not worth worrying about IMO.