All the "Dual Services" I have seen over time were right next to each other [physically!].
I imagine you are describing a system which is derived from a 4 wire Delta [could be closed, could be "Vee"] Transformer bank from the Utility guys. That's the norm of the ones I have encountered. They are as follows:
The 1 phase 3 wire service is separately metered and contains the Grounded Conductor. The 3 phase 3 wire service is separately metered and in it's own gear, as is the 1 phase 3 wire [in it's own gear / separate meter]. Each of these services are fed from the same 4 wire Delta network.
The gear I worked with / around was physically bolted together and bonding jumpers installed [sometimes
] between the sections - as well as within the gear. All bonded to the GES in the 1 phase 3 wire section.
Since they are both fed from the same system, the 3 phase 3 wire section bonds to the system "Neutral", in order to complete the requirements of a Grounded Conductor's action [AKA Ground Faults / Sparks be - a flyin', balls 'O fire, yadda, yadda, yadda].
The only thing I could see being a problem is bonding the Grounded Conductor in two individual services within the same "Area". I am referring to the current loads running on cold water pipes, in the rebar, under the slabs, etc.
If the 3 phase 3 wire system is derived from a separate Transformer, or bank of pots [either from one stand alone 3 phase 3 wire xformer, or 2-3 individual 1 phase xformers connected as 3 phase 3 wire only] and the 1 phase 3 wire system is not a part of the 3 phase system, then there will not be as much of a Grounding Electrode / Grounded Conductor bonding problem.
Now, as far as "Spotting A Meter", the local Utility Design Engineer[s] would be the ones to ask for an accurate answer. If they will allow meters in separate locations, your 1/2 the way home! Then see what the AHJ has to say about that setup. If both agree - your in! If not, back to the drawing board! [AKA both services in same location].
Is there some unresolvable restraints occuring, which is why you + PoCo want to locate the new 3 phase service away from the existing 1 phase service? Like size of the room, etc.??
If it's just to accomidate feeding 3 phase loads easier, maybe you could subfeed from the service and place it next to the 1 phase guy.
If there was a little more information, maybe someone [or I] could give more accurate answers [boy, if that's not redundant
I'll browse the NEC articles 200 - 230 and see what they say.