Thermal and moisture expansion/contraction of the plywood will always want to pull the joints apart. Best result would be to glue together 2 thinner layers of plywood using offset joints. Two layers of underlayment grade plywood such as IntegraPly (5.5mm 5-ply hardwood) will make a very flat, stable floor. Use exterior grade carpenter glue (cheap, easy to spread, and not too thick) and "clamp" by covering the floor with heavy bags for 24 hours. Could be sand, ready mix cement, water softener salt - whatever you might have a use for afterwards. Or, just return the bags.