I was going to ask this as an aside in this question, but I remembered that there are no asides in StackExchange.
Say I've done this again:
I go out and I plant two rows of beets, two rows of turnips, two rows of radishes and two rows of carrots.
Given beets, carrots and radishes like 18 inches between rows and turnips like 24 inches between rows. How close do I plant the first row of turnips to the last row of beets and how close do I plant the last row of turnips to the first row of radishes?
"How to Grow More Vegetables" by John Jeavons, deals with exactly this question. Page 160, "Companion Planting; In Space" -- he's talking about companion planting corn and beets intermingled in a bed, but the same logic applies to your question:
... One way to determine the spacing for different plants grown together is to add their spacing together and divide by 2. If you grow corn and beets together, add 15 inches and 4 inches for a total of 9 inches. Divide by 2 and you get a per-plant spacing of 9 1/2 inches. The beets, then, would be 9 1/2 inches from each corn plant and vice versa.
This is followed by a diagram showing how each corn plant gets the necessary 15" diameter growing space, and each beet gets its necessary 4" diameter space.
No experience here, just some simple thinking and math. I would halve the two distances and add them then use that number. Think about the plant needing half of the row spacing on each side. So if it needs 12" of space then it needs 6" on each side. If another plant needs 18" of space then it needs 9" on each side. Thus if you plant the 12" and the 18" in neighboring rows I would use at least 15" of space between them.
