Neither disc was issued; Sony has a complete run of the ML/MS series, but  
not these catalogue numbers. The Mendelssohn MSND music that was paired on 
LP  with the Schubert WAS issued on a two-track stereo tape, though, so the 
stereo  versions were likely more than the mere figment of someone's 
imagination;  they probably existed at one time.
Rich Kaplan
In a message dated 9/25/2012 3:06:22 P.M. Central Daylight Time,  
[log in to unmask] writes:

From my  database - don't know if the disc was ever actually issued ...  

Mike Gray 
ID Composer Work Number De Bellis 78 Prefix  / Suffix 78 matrix Date 
Initial Label Ensemble Artists Conductor Producer /  Engineer 
146812     GLIÈRE     Ilya  Mourometz, Symphony No. 3 in B minor, op. 42    

11/25/1956    Columbia     Philadelphia Orchestra     
Ormandy, Eugene     

147356    SCHUBERT     Symphony No. 8 in B minor, D759 'Unfinished'  

11/25/1956     Columbia     Philadelphia  Orchestra     
Ormandy, Eugene