Tempered glass is quite strong and a good solution for spans where regular glass would fail. You just want to make sure that there is adequate support across the span. On its side, most of the weight of the shelf and its contents will be on the slots in the particle board ends - this would cause deflection which is the primary point of failure. The deflection is somewhat stabilized by the plastic rail that links the back to the bottom and top (formerly the sides) but this isn't terribly strong because it was never designed for this purpose and isn't itself fastened to anything along its length.
I would reinforce this area for long-term use.
Here are a few things you can do:
Under the bottom of the lower unit you should put a piece of wood along the back directly under the plastic rail, of the exact thickness as the height between the rail and the table top under it. Then you can put one or more sheets of rigid foam (like styrofoam) of a thickness equal to the non-deflected height of the bottom of the glass to the top of the table. That will all but guarantee no issues on that shelf - put your heavier items here.
Next, put a piece of wood similar to the one you just used in the last step between the two shelves running their entire length along the back edge between the two plastic supports of each cabinet. Measure carefully for thickness. You don't want to deflect the glass in either direction as long term deflection can give way to sudden catastrophe. Of course short-term/instant deflection can also lead to immediate catastrophe.
The result of this will be increasing the support of the bottom of the upper cabinet by increasing the strength and rigidity of the support mechanism at the back. This will lead to less propensity to sag which will make for a much happier piece of glass. The weight is being transferred from the shelf to the wood, to the back panel of the bottom shelf, to the wood under that and then to the desk. It is no longer being supported only by the back rail.
Last note: you can find calculators online for safe span distances for tempered glass, you might consult those for additional info.
