One of the things that repeater builders do for best results is use commercial gear. That gear has a higher duty cycle rating than almost all ham gear--but for the 220 band, that kind of gear is almost non-existant. One local repeater in my area also has two Alinco 220 machines back to back in use for an area repeater, but the output power of the transmitter rig has been turned down to less than twenty watts from its maximum. That repeater has also been located in the license trustees shack--where he can check it and disable it at any time.
Bottom line? Yes, it can be done, but it isn't really good to do if the repeater will be at an unattended site. The 220 mhz rigs just aren't as robust or reliable as the commercial gear that makes up the greater part of the 2 meter and 70 cm repeaters in use.
I might also add that two mobile radios co-located (same physical location) normally wouldn't make any sort of repeater worth a darn due to the RECEIVER, which can be desensitized by the local transmitter even with a very good cavity duplexer, and also the TRANSMITTER off-channel noise, which usually isn't reduced even if you turn the power down. The transmitter wasn't designed to be down 100 dB at 1.6 MHz offset, since in normal operation that doesn't matter at all. In repeater operation, it matters.
These rigs don't have any real shielding to speak of and the only thing decoupling them from their DC power source is bypass capacitors. Normally, that isn't nearly enough to keep the TX from getting back into the RX of the other rig.
Transmitter duty cycle would be almost the very last of my worries, as a combination of turning the power down and adding cooling fans could probably keep it reasonably cool. I wouldn't use it without fans.
Then, if it's at a remote hilltop site without heating, the rigs could easily get too cold to even function. A lot of amateur and consumer electronic gear isn't rated for operation below 0C (32F) and a hilltop repeater site in most places can get much colder than that.
This is why "real repeater gear" costs more. It's designed for greater thermal extremes, is adequately heatsinked and cooled to permit continuous operation; the TX and RX and in very well shielded, separate compartments with DC feedthrough LC networks to isolate them both from the power source; the RX usually has a helical resonator in the first RF stage to start rolling off anything outside the band (high or low side); and the transmitters are designed for a reduced off channel noise spectrum. They're also usually more frequency stable over temp extremes. Amateur synthesized gear can drift in frequency quite a lot between 0 and 100 degrees F, while repeaters shouldn't and usually don't.
