I dont know, Thats a tuff one. I would of cut it with no problem without the tank sitting on top of it but its hard to say with the tank there. If you really want to cut it which i would probably go threw with it is take the shelf out and cut it down about 2 inches. then i would take temp 2x4's and place them verticaly under the tank in the stand to help take the load off. Now once that is complete i would build a wood frame out of 2x4's and place them on the side you want to cut. The frame would be up the back and up the front. But try and make the 2x4's tight enuff where you would need a hammer to lightly tap them into place and then screw them into place. Also either treat the wood or buy treated. Then mark out where you want to cut and cut.
But there is alot to look at here.
1. Is the shelf helping the sturdiness of the tank?
2. Do you want to put this much work in it?
3. Will the wood start splitting or cracking while you are cutting it?
Its really a hard chore and it would take a full day to do...
I guess you could always just place two seperate containers or tanks in the bottom and plumb them together. which would be a nice little
DIY project as well.
I wish i could help you more.. Could you set a tank next to it or maybe behind it?