I don't have these but could make a guess. I have not had luck with peach/nectarine pollinating plum or plum hybrids. DWN describes SpiceZee as a nectarine interspecific and if you look at the published patent, the parentage is mostly nectarine/peach. Its physical characteristics seem to be very much nectarine as well (appearance of fruit, seed, flowers, etc., and is self-fertile). My guess is that it would not pollinate a plum or plum hybrid.
Since you have only enough room for the two trees, would it be possible for you to graft another variety onto a branch of the pluerry for pollination? Burgundy is recommended by DWN but other plums or plum hybrids will likely work as well.