Sean, it will probably hold up fine but I don't know about the long run though. Have you consider Gorill Rack? They're built much more sturdy and looks better. Orchard OSH carry a full line of Gorilla Rack.
I would not trust my fish on that rack. I have one that I bought from Sam's Club and it holds well for soda and dry good. That plastice wedge is plastic and it could break. The gorilla rack that you can also buy at Sam's club is more sturdy, but its longer. I think the best way is 2x4's like mentioned.
I met someone that used this rack for 3 55 gallon acrylic when I was buying a tank. I cannot recall his screen name but hopefully he will see this thread and reply. I'll send you his SN if I remember.
I do not think it is a matter of whether or not the cart will hold them. It is really about the amount of flex that there will be when you put the GLASS tank on there. I use these all the time. My weight alone will cause the shelf to bend. This is a bad idea. Go with gorilla racks.