Easiest way to expand the trellis is to put more 6 ft stakes on either side and secure a sturdy cross beam (I use 3/4-7/8" bamboo) with zip ties at the top. I tie on nylon "vegetable trellis" or zip tie on wire fencing, or even just a few rungs of jute string. (Tomato vine stems can be wound on the string as they grow. Other climbers will hold on on their own.)
Prune off side shoots and branches that are growing in "wrong direction" -- outward away from the trellis, interfere with each other, or overlap -- so they don't densely shade each other. Open up enough space for good air circulation. Start spraying thoroughly with milk solution.
At some point, if you live in humid area, you will need to sacrifice one or the other since all of them can suffer from fungal disease. Pea vines first, I think. probably cucumbers next, unless your tomatoes are determinate variety.
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