Buying 2 4x2 tanks and modifing in the described manner would likely be a no no since the glass used is not designed for the increase pressure. Not just the sides but also the bottom panel.
Obtaining two 4x4 craked tanks is another matter
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Taking 2 4x2 tank and connecting them via drilling a large hole and using tubing or pvc with bulkhead in that manner would work since there is no increase in pressure on the glass (be aware that tempered glass is not suitable for hole drilling).