Look at a picture of the EX2300-C, and then look at a picture of the SRX300. Made for each other . . .
They effectively have the same physical design down to the corner pieces that appear to facilitate physical stacking (maintaining an airgap) outside a rack. Recall these are both fanless units, and promoted as well suited to working together in a branch situation.
Now check the physical measurements, how is one to arrange a SRX (12.63 x 1.37 x 7.52) atop a EX (10.98 x 1.72 x 9.4) or vice-versa? There is no way that the corners are going to align, unless the EX has a cut out that allows it to overhang the SRX at the rear, which I can't see, in pictures.
However, given the recent improvements in how the SRX300 may be configured, and the relative age differences between it and the EX2300, one can only hope similarly enhanced capabilities are brought to the switch as well (real soon now). Just because branches are small it does not mean that they do not have sophisticated requirements.