I would think the humidity should be higher. Placing many plants close together will usually boost it. How close is your HPS light from the tops of the tomato plants? The tops of the cukes? I use High Output T5 fluros and I leave them on for 18 hours a day, but I'm not sure if that conversion would work with a more powerful HPS unit in terms of the amount of mols/day. You should water on a very consistent basis. Record your waterings, and how much you gave the plants. Plants like consistency more than anything else. What is your fertilizer regimen and potting mix for both plants?

http://pollinator.com/tomato.htm

http://greenculturesg.com/articles/mar06/march06_feature_2.htm

I fertilize once a week with an all purpose (agrotek) liquid organic fert. THe plants are in 5 gallon bags. The mix is, roughly, 60% black compost ("Sea Soil"), 30% #4 (peat & perlite), 10% worm castings, top dressed with a Turkey Trot (calcium enriched) product called ActiSol. I think you are ultra right about the regular waterings. I think that the humidity is ultra low. I'm going to try a mass pebble tray by filling the 2x4 drain table with lava rocks and placing the pots on top of those, and putting a shutoff valve on the drain to keep some water in the table . . perhaps . . I love summer.

Once a week is too much fertilizer unless you are diluting it to 1/4 strength and cucumbers use more nitrogen than tomatoes IME. Compost isn't good in containers. The containers need to drain. What is #4 peat & perlite?? You're making it way too complicated. They don't need high humidity and aren't that picky about soil but it must drain.