an inch an a half is fine between the tires.... as for the comment about "what if something gets kicked up between them?".... well, what if it happens right now the way they are?... its going to get kicked out of the way because one tire face is going up and the other is going downward, so it cancels each other out.. but gravity is still in your favor so the debris will never get caught between the two opposing directions and go into a perpetual spin between the tires...
yes, the tires can move a little, and the center axle of a tridem much more so than a tandem axle set up. but its going to be a rare instance where in normal operation the axle moves over an inch fore and aft.... and this "rare inch" of movement includes a half inch forward from normal to a half inch rearward from normal.... any axle that can move a full inch rearward/forward from is normal location means the suspension is faulty.