Sorry i was not more descript, I was thinking more along the lines of having a repeater, and if i had it set to repeat on channel 1, can i switch to channel 2 and have the same repeater set to repeat on channel 2 at the same time it's repeating on channel 1? or are 2 repeaters required for multi channel repeating? the reason i ask is because i see that some of the newer repeaters like the Mototrbo's have more than 1 channel, and i've seen 16 and 32 channel vertex repeaters as well
The general rules are 1 repeater will take up two frequencies (an input frequency that the handhelds, mobiles, etc. transmit on and the repeater listens to, and an output frequency that the repeater transmits on and the handhelds, mobiles, etc. listen to). In the old analog world, 1 repeater = 1 channel. You want more channels that are repeated, you license an additional pair of frequencies for each additional channel. To define a "channel on these you simply need to specify what frequency is assigned to that channel.
On the PL issue, different PLs don't define different channels simply because the transmission, regardless of what PL is used, takes up the whole channel so only one is available at a time. This is like an old pay phone, while that phone can call anyone in the world, only one person at a time can use it.
Some newer technologies allow you to split a frequency pair into more than a single channel. This is by that technology somehow splitting the signals up so they no longer take up the whole frequency. For MotoTRBO, this is done by what can be viewed as a time-share arrangement (yes, like those condos at the beach where you own one week and other owners own their own week). MotoTRBO splits the channels into two very short time slices, both on the same frequency. Channel 1 may use timeslice "A" and channel 2 may use timeslice "B". A "channel" on those systems is defined not only by what frequency it uses, but also by what timeslice on that frequency carries that channel.
Trunking does things a bit differently. A single frequency pair (one repeater) carries only a single transmission at a time. The difference is that a trunking system uses the idle time to allow other transmissions to take place. A "channel" on a trunking system isn't necessarily assigned a specific frequency, but a talkgroup. A radio wanting to transmit on a "channel" first must ask the trunking system for permission to talk on that channel/talkgroup and the system will tell the radio what frequency to use for that transmission. If no frequency is currently available, the system sends out a cancel code to the radio, which generally gives a beep (often more of a bonk) so the user knows to try again in a bit. This is how a simple 8 or 9 frequency trunking system can handle many times more "channels" than there are frequencies on the system. If, like on most systems, the activity on a single channel is generally quite low, more "channels" can be assigned onto that system.
The RR WIKI has articles that may help explain many of these topics so I've only given a brief discription of how they work.