1 Frequency-agile systems will be a better choice.  If the wireless equipment will be used in situations where several other wireless systems are likely to be present, UHF systems are recommended. This is because more frequencies are available, reducing the chances of interference.
http://www.audio-technica.com/cms/site/af9d127e26abf498/
UHF Versus VHF. The idea that UHF wireless systems are inherently better than VHF systems is widespread. But while UHF does offer many advantages, its not automatically the better choice, especially now as UHF bands become more crowded and remain mired in regulatory uncertainty.
http://www.audio-technica.com/cms/site/af9d127e26abf498/
Shure Dual Handheld UHF Wireless Microphone System BLX288/PG58. New pyle 2 channel UHF 2 Microphone System w/ Rechargeable Dock LCD Display. Premier Series UHF Wireless Microphone System with (2) Handheld Mics, Dual Rechargeable Dock, Selectable Frequency, LCD Display, Rack Mountable.
http://www.ebay.com/bhp/wireless-microphone-uhf
1 This is because most channels are used in one location or another, and frequency conflicts will eventually occur. Frequency-agile systems will be a better choice.  If the wireless equipment will be used in situations where several other wireless systems are likely to be present, UHF systems are recommended.
http://www.audio-technica.com/cms/site/af9d127e26abf498/
And some digital systems use frequencies within the 2.4 GHz band to avoid interference from TV stations and cell phone towers. Digital Wireless Microphones: Digital wireless systems convert audio to a digital signal before sending it through the air.
http://www.zzounds.com/lp/wireless-microphone-system-buying-guide/46
The Basics -- Transmitters and Receivers: Every wireless mic system has two components: a transmitter, typically worn or held by the performer -- and a receiver, which collects the radio signal from the transmitter, converts it to audio, and sends the audio through a cable to your mixer or power amp.
http://www.zzounds.com/lp/wireless-microphone-system-buying-guide/46
For two-mic setups, it's a good idea to choose a Dual system that's designed to run both microphones simultaneously. VHF vs. UHF vs. 2.4 GHz: Most wireless systems operate in the Very High Frequency or Ultra High Frequency bands. Due to their shorter wavelengths (300 - 3000 MHz), UHF systems are less susceptible to drop-outs or dead spots, even over distances of 200+ feet.
http://www.zzounds.com/lp/wireless-microphone-system-buying-guide/46
But while UHF systems are less likely to suffer interference, cost considerations tend to favor the use of VHF systems. Still, interference and cost are just two matters to consider. Whether the choice is UHF or VHF, diversity systems are highly recommended.
http://www.audio-technica.com/cms/site/af9d127e26abf498/
If you need to mic up multiple singers, dual systems give you two microphones and a multiple-channel receiver. Analog UHF and VHF Wireless Handheld Systems: AKG WMS40 MINI2 Dual Vocal Handheld Microphone ...
http://www.zzounds.com/lp/wireless-microphone-system-buying-guide/46
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which wireless mic is better uhf or vhf
The wireless mic ultra high frequency is better than the very high frequency. Ultra high frequency wireless mic is better than very high frequency.