I would unclip your two DI housings and orient them both vertically with flow from the bottom to the top. I'd do this now. Change the resin only when needed. Add resin now if you note that the tubes are not 100% full/packed tightly. If you need clips to mount them vertically we have clips for 2" and for 2.5" tubes.
The only place we see horizontal DI stages is on systems built by low end ebay vendors. Typically they are drinking water RO systems assembled overseas, and the local vendor slaps on a couple of clear horizontal housings (intended for GAC typically) and calls it an RODI. When we see this configuration it immediately calls into question the qualifications of the vendor.
Russ