First: if your "tank" has a reptile thermometer sticker on the inside, then it is probably a glass reptile cage, which is NOT an aquarium. Glass The glass used on glass reptile cages is thin and can't handle the weight and pressure that water puts on it. Check to see if the glass in your "tank" is as thick as a real aquarium of the same dimensions, and if it's thinner DON'T use it.
In theory the design you propose will work fine, as long as each siphon can keep up with the flow provided by your return pump. You are letting gravity get your water to its lowest point, and using a single return pump to get it back to the top - so no problem.
That being said, I don't trust siphon type overflows. If bubbles gather in your siphon tube, or your siphon stops working for any other reason, then you will have an overflow, and your pump will run dry. With two siphons you are doubling the chances of having this kind of problem. I would play it safe by drilling each tank, and using overflows that don't depend on a siphon. If you have downstairs neighbors, a flood of only a few gallons could cause $2000 or more in damage to their ceiling. If a small flood is no big deal for you, then go for it. Otherwise, I don't recommend that you take the chance with siphons, but that's me.