Cannot start 2 instances of OnLine 7.x

Until recently, we ran our test database on OL 7.x and our production database on OL 5.x with no memory conflicts. Then I upgraded the 5.x to 7.x (dbexport, dbimport, etc.), and got it running. During the upgrade, I did not have the 7.x engine running, but I did have a level 0 archive of it that had been successfully restored a couple of times. Now that my production database is up, I cannot get my test engine to come up. When I do an 'oninit', it returns to the command prompt OK, but within seconds states: oninit: Fatal error in shared memory initialization The log file contains: Wed Oct 8 13:15:45 1997 13:15:45 DR: DRAUTO is 0 (OFF) 13:15:47 INFORMIX-OnLine Initialized -- Shared Memory Initialized. 13:15:47 Physical Recovery Started. 13:15:47 Physical Recovery Complete: 0 Pages Restored. 13:15:47 Logical Recover Started. 13:15:50 Logical Recovery Copmplete. 0 Committed, - Rolled Back, - Open, - Bad Locks 13:15:50 INFORMIX-OnLine Stopped. 13:15:50 mt_shm_remove: WARNING: may not have removed all/correctsegments Furthermore, onstat -g seg for the production engine shows: Segment Summary: (resident segments are locked) id key addr size ovhd class blkused blkfree 214 1385973763 c9eeb000 2744320 648 M 332 3 808 1385973761 c7f61000 41410560 1484 R 5051 4 609 1385973762 ca6df000 67108864 1628 V 5813 2379 Our system has 512MB of ram, of which a minimum of 120MB has been free when I've tried to start the second engine. Any hints on running two engines simultaneously would be much appreciated!