If you are eliminating "hole through the side simple overflow" then you have probably arrived at two pumps, with the return pump in a standpipe/sump which has a simple overflow into it to the maintain the level in the plant side, and is operated by a float switch on the sump/standpipe. Managing the return with a siphon is even more likely to cause a flood than drilling holes in the tanks.
I'd drill the hole for the overflow, and then use rigid pipe of generous size (since you only have 2" of head you need a large pipe) for the return attached to it, as I consider the hole with a pipe near the top of the tank far less risk than one pump failing and the other pump merrily overflowing. Or some cockamamie siphon setup failing and the pump merrily overflowing.