The Spiritualist's Phantom is described as self-involved and jealous and therefore does not co-exist with other summoned creatures. The ability specifically mentions that "a phantom refuses to manifest in the presence of an eidolon or shadow summoned by the phantom's master".
Eidolon is a pretty obvious reference, but what is Shadow in reference to? I checked what I consider the obvious possibilities (alternative Summoner class features via archetypes in such mainline books as the UC and the ACG) and found nothing. So what is it supposed to be referring to?
A Shadowdancer's Shadow, I would imagine
Probably the shadow dancer prestige class.
It is something that you can gain by the 3rd level of that class, and it uses your stats to determine everything that scales. So hell, it seems more viable than an eidolon as something to dip while going main as spiritualist.