They are designed to be set out at 90deg to each other. I run one up the rollover mast and one across the servo parallel to the ground in my 12th scale. Like Chevy, no issues.
I've seen both run vertical, both run horizontal and even with one removed from the receiver - only one aerial in use. At the range we have it doesn't seem to make any difference. For guaranteed performance, one vertical, one horizontal. HTH
