How can I immobilise a non-T&G subfloor?
While t+g might have been a bit better, as long as your sheets landed well (1/2 on, 1/2 off) the joists, it'll be fine.
If you're still concerned AND you can get underneath, you could install solid blocking (2x4 on the flat) perpendicular to the joists under the seams.

Edit to reflect new information (seams not on joists):
Soooo... this isn't great, but still not the end of the world. If you run 3/4&quot; t+g pine flooring perpendicular to your floor joists, you'll get a little flex, but your plywood underneath won't saw itself to death.
If you really wanted to fix this, you'd cut out the ply in the joist bays where the seams are. Cut next to the joist, so that the plywood that stays behind is on the entire joist. Attach something (could be as small as a 2x4, and doesn't, strictly speaking, have to be continuous) to the sides of the existing joists, and set a complete section of plywood on that. Glue/screw/etc. Use a 2x4 on the flat if you have any seams perpendicular to the joists.