Nive Enterprise setup!; but....wow, two Atoms!

Btw, if you feel comfortable with the solder iron, you can replace the IC over the Z80 in the lower right corner of the PCB with an EPROM 27C512. This way you can upgrade EXOS to the last version (I suppose Zozosoft already burnt this one for you) and merge it with IS-BASIC, freeing one of the ROMs in the cartridge.
Another benefit is that you'll never need again a cartridge with BASIC to boot the machine. No stress to the cart port!

Keep an eye to the 512KB RAM upgrade kits, right now it's the best addition to your EP64!