The minimum separation between the UHF and H-VHF would be about 3', more is better but after about 5' there's very little interaction.
There's not a hard fast rule re. which antenna should be on top... Go with what works best in you application. If there's no difference in reception reliability, the smaller (usually the UHF antenna) wind load would go on top to reduce the stress on the tower/mast.
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Help with Reception and include a link to the TV Fool reception prediction for the specific location. Thanks